Introduction
Of late, a novel paradigm has emerged, providing a useful framework for assessing environmental chemicals and human health inclusive of fecundity and fertility. This paradigm, sometimes referred to as the Barker hypothesis, the early or fetal origins of disease, the thrifty phenotype or the developmental origin of disease and health, posits key elements that challenge past thinking about disease etiology. These include recognition that: (1) human development is more than the unfolding of the rigid genome toward one of developmental plasticity; (2) exposures during critical and sensitive windows may affect disease risk across the lifespan as a result of epigenetic-induced changes whose original intent may have been to allow the embryo/fetus to adapt by altering organogenesis for anticipated needs in later life; (3) the complexity of disease such as gene–environment interactions including the potential for low-dose additivity or synergy of chemical mixtures; and (4) the interrelatedness of health states such as fecundity, gravid health status and later onset adult disease.Reference Barker1–Reference Heindel5 In response to this evolving paradigm, innovative methodologies have emerged, such as the life course epidemiological method for assessing a multitude of exposures and diseases across the lifespan.Reference Ben-Shlomo and Kuh2 In addition, Bayesian models appropriate for the hierarchical data structure that arise from such paradigms along with joint models permitting the assessment of more than one health outcomeReference De Stavola, Nitsch and dos6, Reference Carlin and Louis7 offer analytical techniques for researchers interested in life course epidemiological research.
Worldwide, there is speculation that human fecundity is declining and that current fertility (live birth) rates are unlikely to sustain populations.Reference Lutz, O'Neill and Scherbov8 Some authors speculate that environmental agents are responsible, in part, given the rather rapid onset of such declines in the absence of detectable genetic changes in human populations, whereas other authors argue that social forces are responsible. A collective body of research focusing on the early origins of male fecundity and later onset disease has been thoughtfully synthesized and articulated as the testicular dysgenesis hypothesis (TDS). Moreover, the TDS hypothesis notes the sentinel role of fecundity endpoints as an overall marker of health status and eventual disease risk.Reference Skakkebaek, Rajpert-De and Main9 Briefly, the TDS hypothesis suggests that changes in semen quality,Reference Andersen, Jensen and Carlsen10, Reference Carlsen, Giwercman, Keiding and Skakkebaek11 genitourinary (GU) malformationsReference Chilvers, Pike, Forman, Fogelman and Wadsworth12, Reference Paulozzi, Erickson and Jackson13 and testicular cancerReference Moller14 may share a common in utero etiological pathway.Reference Skakkebaek, Rajpert-De and Main9 In fact, the interrelatedness of adverse male reproductive outcomes from birth through adulthood has long been reported in the literature. For example, GU malformations are associated with diminished adult fecundity as is testes cancer, even in the pre-diagnosis period.Reference Baker, Buck, Vena and Moysich15, Reference Prener, Engholm and Jensen16 This body of evidence is evolving in the context of data supporting an important etiological role for periconceptional and prenatal factors in the development of many adult onset diseases such as type 2 diabetes or cancer.Reference Leon, Lithell and Vagero17–Reference McCormack, dos and De Stavola19 Support for the TDS hypothesis is augmented by recent recognition that such exposures may permanently and irreversibly reprogram the developing organism for adult onset disease.Reference Barker20 Experimental evidence supports a relationship between anti-androgenic exposures during sensitive intrauterine windows and a spectrum of adverse reproductive and developmental effects.Reference Wilson, Blystone, Hotchkiss, Rider and Gray21
Recent authors have summarized the available literature regarding environmental influences on female fecundity and fertility in relation to the purported ovarian dysgenesis syndrome (ODS).Reference Buck Louis and Cooney3, Reference Mendola and Buck Louis22 First postulated by Buck Louis and Cooney,Reference Buck Louis and Cooney3 the ODS hypothesis recognizes the many similarities in the embryonic development of the male and female reproductive tracts while borrowing heavily from the TDS paradigm. Although evidence supporting the TDS paradigm is well under way, empirical support for ODS is still evolving as researchers begin to assess endocrine disruptors in relation to fecundity, gravid health and later onset diseases from a life course perspective.
For purposes of this paper, we posit that genetic and environmental factors may impact the structure and function of reproductive organs and tissues manifesting in a spectrum of outcomes. As illustrated in Figure 1, alterations in ovarian development may manifest as structural or functional changes including embryogenesis, folliculogenesis and steroid signaling, respectively. A spectrum of possible effects is possible, including birth or müllerian anomalies (MA), anovulation and altered menstruation, respectively. Speculations that fecundity impairments and gynecologic disorders may increase cancer risk possibly via altered epigenetics underscore its inclusion in the figure, but with no predetermined directionality. We further conceptualize ODS as arising from exposures during the periconception or in utero windows and/or possibly transgenerational effects such as in the case of diethylstilbesterol (DES) exposure.
Fig. 1 Ovarian dysgenesis paradigm.
We define fecundity as the biological capacity of women for reproduction,Reference Wood23 irrespective of pregnancy intentions. We define environmental exposures as all non-genetic factors, but specifically focus on environmental chemicals with a purported ability to disrupt endocrine pathways or so-called endocrine disrupting chemicals (EDCs). This paper provides an overview of the biological plausibility of the ODS conceptual paradigm followed by a review of the available human evidence in support of an early origin for female fecundity, gynecologic health and later onset diseases.
Biological plausibility of ODS
The vulnerability of the developing conceptus to a spectrum of environmental exposures is well established including exposures arising in the periconception period or the interval before, at or shortly after conception.Reference Chapin, Robbins and Schieve24 Critical windows of human development are characterized by high rates of cellular proliferation and changing metabolic capabilities.Reference Calabrese25 Xenobiotic exposures occurring during the early embryonic period of rapid development and beyond may adversely affect the development of the gonads and female reproductive tract (i.e. uterus, cervix, fallopian tubes and vagina) with implications for female fecundity and, possibly, adult health. Kuh and Ben-ShlomoReference Kuh and Ben-Shlomo26 have eloquently articulated the need to distinguish between critical and sensitive windows, given that exposure during the latter may still adversely affect development including adult onset diseases, though possibly with reduced magnitude. For example, an exposure during a critical embryonic window may produce structural birth defects, whereas those occurring outside the critical window may produce other nonstructural or functional deficits. This recognition challenges investigators to look beyond structural defects in considering functional changes that may manifest as a spectrum of adverse effects across the lifespan.
Delineating critical and sensitive windows is important for research focusing on the role of epigenetics in human reproduction and development in recognition that embryonic development is regulated by both genetic and epigenetic information. Genetic mechanisms are largely dependent upon the DNA code,Reference Lucifero, Chaillet and Trasler27 whereas epigenetic mechanisms reflect molecular alterations in gene expression or phenotype without a change in DNA sequence.Reference Latham28 Considerable epigenetic reprogramming occurs during gametogenesis and early embryogenesis,Reference Obata, Kaneko-Ishino and Koide29 and exposures including assisted reproductive technologies during this window may result in disturbances in epigenetic processes with a spectrum of adverse health effects across the lifespan that may either be transitory or passed through the germline to the offspring.Reference Owen and Segars30
Epigenetic reprogramming may affect both structure and function. During critical and sensitive windows, the female gonads and reproductive tract are vulnerable to a host of xenobiotic agents during embryonic development, which commences at approximately 4 weeks post-conception. Recognition of this and other developmental windows is important for assessing and interpreting the ontogeny of MA, gonadal disorders or reproductive site cancers. Development of the female reproductive tract requires completion of a series of highly interrelated and timed processes, including müllerian duct elongation, fusion, canalization and septal resorption, which extend well into the second trimester. Exposures at any one of these stages may affect the likelihood of malformations or other adverse outcomes.Reference Dorais and Peterson31, Reference Witschi32 Since the reproductive and urinary tract develops during similar windows and often interdependently, interpretation of MA should be performed in conjunction with an assessment of urinary malformations.Reference Acien, Acien and Sanchez-Ferrer33 Recent studies on MA suggest a strong familial aggregation and follow a polygenic and multifactorial inheritance pattern consistent with the ODS paradigm.Reference Hammoud, Gibson and Peterson34
Ovarian development begins approximately 4 weeks post-conception with the migration of embryonic germ cells from the yolk sac to the primitive gonadal folds where they undergo mitotic expansions and become oogonia.Reference Witschi32 At approximately 8–13 weeks’ gestation, the oogonia enter meiosis and then remain in a protracted state of meiotic arrest surrounded by a layer of granulosa cells until just before ovulation, in response to the pre-ovulatory gonadotropin surge.Reference Gondos, Westergaard and Byskov35 Primordial follicles in their arrested state represent another window of potential vulnerability to a host of toxicants, with the potential for permanent reprogramming of the organism. Similarly, reproductive tissues that are eventually responsive to ovarian steroids also represent vulnerable structures during this period in the context of ODS. Embryogenesis and gametogenesis are highly influenced by steroid hormones raising concern that endocrine disruptors may be capable of disrupting these processes with implications for both the current and future generations.
Despite the considerable interest in epigenetics that has materialized from mapping the human genome, it is noteworthy that the concept of epigenetics is not new per se. Rather, it was first coined by the pre-eminent developmental biologist, Conrad Waddington, in the 1940s following the blending of two concepts, that is, epigenesis + genetics = epigenetics.Reference Van36 Epigenetics refers to the chemical modification of DNA and chromatin that may affect genomic functioning possibly resulting in altered disease susceptibility across the lifespan. The prototypical chemical exposure for epigenetics is DES, though considerable evidence suggests a similar role for bisphenol A (BPA), as recently reviewed.Reference Heindel5 Endocrine disruptors modify chromatin packaging resulting in modification of histones and promoting DNA methylation. As such, modifications may turn on and off gene expression patterns. Exposures during the earliest stages of human development may thereby have lifetime implications for health and disease.
EDCs and ODS
EDCs include naturally occurring estrogen-like exposures such as the phytoestrogen genistein or synthetic agents such as BPA, dioxins such as polychlorinated dibenzodioxins and polychlorinated dibenzo furans, organochlorine pesticides such as dichlorodiphenyltrichloroethane (DDT) or its metabolite dichlorodiphenyldichloroethylene (DDE), phthalates and polychlorinated biphenyls (PCBs) and others.Reference Peterson, Carrell and Varner37 Experimental and observational human data suggest a role for EDCs in broad categories of human health – fecundity, gravid health status and later onset adult diseases. For example, human uterine tissue contains several different types of cells that express steroid receptors making them potential targets for EDCs including the endometrium, which represents the end organ of the hypothalamic–pituitary–ovarian axis. The structure and function of the endometrium are delicately regulated by ovarian sex hormones (estradiol and progesterone) and are sensitive to small disturbances, such as those induced from chemical exposures. Endometrial blood vessels are critical for implantation and placentation during early pregnancy. Endothelial cells covering the luminal surface of blood vessels control vascular morphology and function via angiogenesis, vascular remodeling and functional changes. For example, PCB congeners nos 77 and 126 compromise the normal function of porcine vascular endothelial cells by increasing expression of CYP1A1 gene, oxidative stress and DNA-binding activity of nuclear factor kappa B.Reference Hennig, Meerarani and Slim38 Animal evidence shows that vascular endothelial growth factor (VEGF) secretion is decreased by 2,3,7,8-TCDD in chick embryos.Reference Ivnitski-Steele, Sanchez and Walker39 In rodent studies, BPA induces VEGF expression in the uterus, vagina and pituitary.Reference Long, Burke, Bigsby and Nephew40 Human endometrial endothelial cells (HEECs) express estrogen receptor beta (ERβ) and progesterone receptor that are controlled by cyclical changes in estrogen and progesterone concentrations during menstruation and pregnancy. Expression of steroid receptors might render HEECs susceptible to EDCs. Bredhult et al.Reference Bredhult, Backlin and Olovsson41 assessed the in vitro effects of several EDCs (DDT, PCB 77, PCB 126, di-n-butyl phthalate, BPA, 2,3,7,8-TCDD) on proliferation and viability of HEECs, and noted that some EDCs affected cell viability and increased the proportion of necrotic cells. Therefore, it is possible that EDCs may have both in vivo and in vitro effects and influence processes involving endometrial angiogenesis.
Sex steroids, the aryl hydrocarbon receptor, cytochrome p450 enzymes and thyroid homeostasis are the major targets of EDCs underscoring the potential for a spectrum of reproductive and developmental impairments dependent upon a variety of factors such as the number and target ligand(s) of the EDCs, along with route of exposure, timing and dose considerations, host susceptibility and other lifestyle or behavioral factors such as stress, nutrition and lifestyle. For example, a significantly higher percentage of abnormal metaphase II-arrested eggs was observed in mice exposed to BPA, which were fed soy in comparison with animals not fed soy, suggesting that BPA's effect on growing oocytes may be modulated by dietary phytoestrogens.Reference Muhlhauser, Susiarjo and Rubio42
EDCs may also function as agonists by mimicking the activity of endogenous hormones leading to the expression of estrogen-responsive genes by binding to the ER (e.g. DES, genistein), or as antagonists by inhibiting receptor activity such as inhibiting the binding of ER-coactivators to prevent transactivation. EDCs have transgenerational effects including transmission through the paternal line, as in the case of DES; the transgenerational effects may be more profound than those observed in the first generation.Reference Jirtle and Skinner43–Reference Walker and Haven47 Transgenerational effects are defined as the acquired phenotype or the disease's ability to be transmitted to subsequent generations through the germ line without direct exposure to the toxicant.Reference Skinner45 Thus, EDCs are of concern to both exposed individuals and subsequent generations, irrespective of their own exposures.
Human evidence for ODS
Female fecundity
Within the framework of ODS, several recent papers have reviewed the purported reproductive and developmental toxicity of EDCs and underscore the importance of assessing effects across the lifespan in keeping with the continuum of female fecundity.Reference Buck Louis, Lynch and Cooney48–Reference Toft, Hagmar, Giwercman and Bonde52 A woman's reproductive years are typically defined as 15–44 years, though considerable variability exists at the population level. Recent data suggest that the reproductive lifespan has increased from 36.1 to 37.7 years for women born between 1915–1919 and 1935–1939, respectively,Reference Nichols, Trentham-Dietz and Hampton53 possibly due to faster generalized growth,Reference Karpati, Rubin, Kieszak, Marcus and Troiano54 an earlier age at menarche,Reference Herman-Giddens, Slora and Wasserman55 or a later age at natural menopause currently estimated to be 51.4 years for US women.56 Although the implications of a longer reproductive life are speculative and include a higher risk of reproductive site cancers, considerable interest exists in identifying exposures or mechanisms that may impact either end of the reproductive lifespan.
The importance of the in utero environment and adult female fecundity is now recognized, especially given a growing literature supporting a relationship between fetal growth restriction and ovarian development and function. For example, girls born small-for-gestational age are reported to have poorer ovarian development, diminished follicle stimulating hormone responsiveness and increased anovulatory cycles than adequately sized girls at later ages, thus supporting a role for in utero programming.Reference de Bruin, Dorland and Bruinse57–Reference Ibanez, Potau and Ferrer59 We are unaware of any studies that have measured in utero chemical concentrations in relation to subsequent fecundity, most likely a reflection of the two plus decade follow-up that would be necessary. However, a few in utero exposures have been assessed in relation to time-to-pregnancy (TTP; which is a measure of female fecundity) and the secondary sex ratio (ratio of male-to-female births). In utero exposure to cigarette smoke was associated with a decreased fecundability odds ratio for the woman later in life denoting a longer TTP.Reference Jensen, Joffe and Scheike60 In addition, maternal preconception PCB concentration was associated with a reduction in the secondary sex ratio reflecting a female excess of live births.Reference Taylor, Jackson, Lynch, Kostyniak and Buck Louis61 Other authors have reported reductions in the secondary sex ratio for PCBReference Weisskopf, Anderson and Hanrahan62 and dioxin exposures.Reference del Rio Go, Marshall, Tsai, Shao and Guo63, Reference Mocarelli, Gerthoux and Ferrari64
A few papers have assessed EDCs and TTP . A longer TTP or diminished female fecundity has been reported for select PCB congeners in both prospectiveReference Buck Louis, Dmochowski and Lynch65 and retrospectiveReference Law, Klebanoff, Brock, Dunson and Longnecker66 cohort studies. Prolonged TTPs have been reported for other EDCs, including the retrospective capture of TTP and DDE,Reference Law, Klebanoff, Brock, Dunson and Longnecker66 dioxin,Reference Eskenazi, Warner and Marks67 polybrominated diphenyl ether or PBDE congeners,Reference Harley, Marks and Chevrier68 and the prospective capture of TTP and mercury.Reference Cole, Wainman and Sanin69 To our knowledge, no prospective cohort study has measured EDC exposures during critical and sensitive windows of human development.
In the United States, some evidence exists to support an increase in the temporal pattern of fecundity impairments or infertility among women of reproductive age. Using data from the National Survey of Family Growth, the percentage of women with both disorders increased from 9% to 13% in 1988 and 1992, respectively.Reference Stephen and Chandra70 However, when restricted to married women, prevalence decreased largely ignoring temporal increases in cohabitation rates and births to unmarried women.Reference Stephen and Chandra71
Fertility patterns in the 21st century are declining throughout much of the world, possibly the result of women having fewer children or delaying births to later ages.Reference Daguet72 However, environmental influences on fecundity may account for some of these observed patterns assuming that fecundity impairments reduce unintended pregnancies or completed family size. In addition, the inverse relationship between age and fecundity may reflect a loss in the number of quality of germ cells.Reference Leridon73, Reference Menken, Trussell and Larsen74 Without querying women about their time at risk for pregnancy, it is difficult to fully interpret the declining fertility rates observed throughout the world. For example, it would be informative to know whether completed family size required 3, 5, 10 or more years of trying. This issue has prompted some investigators to articulate a need to monitor fecundity either by prospectiveReference Olsen and Rachootin75 or retrospectiveReference Joffe76 surveillance of TTP. Opponents argue that the multitude of social factors impacting fertility precludes our ability to delineate environmental causes.
GU malformations
GU malformations can impact fecundity or fertility and may arise in utero either through time-sensitive or transgenerational exposures. Unlike male GU malformations that are widely reported in the literature, proportionately speaking, there appears to be a smaller literature on female GU malformations from a temporal perspective. Accurate incidence or birth prevalence estimates for female GU malformations are difficult to obtain except for geographic areas served by birth defects registries. Published data often exclude minor GU malformations or rely on passive reporting mechanisms or do not stratify rates by infant sex. Despite the inclusion of MA as a part of clinical examinations among reproductive medicine clinics, its true incidence and prevalence remain lacking at the population level. This reflects incomplete recognition and reporting, non-uniform classification systems, varying diagnostic approaches, asymptomatic nature, unclassified meosnephric anomalies and/ or differing study populations. Although plausible, assessing MA in relation to EDCs is challenging in light of these methodological limitations.
A few publications have focused on EDCs and GU malformations in girls, particularly given the evidence for DES and related structural defects (e.g. T-shaped uterus with or without a small uterine cavity and structural abnormalities of the cervix).Reference Herbst and Bern77, Reference Kaufman, Adam, Binder and Gerthoffer78 A recent review of the malformations literature in three different populations reported the prevalence of congenital uterine anomalies to be 6.7% in the general population, 7.3% in the infertile population and 16.7% in the recurrent miscarriage population.Reference Saravelos, Cocksedge and Li79 These findings underscore the importance of choice of study population for assessing EDCs and GU malformations, and provide some evidence for a possible shared etiology for malformations and later reproductive impairments.
Uterine fibroids
Uterine fibroids (uterine leiomyomas) are benign tumors that originate from the smooth muscle layer of the myometrium and the accompanying connective tissue of the uterus.Reference Dorland80 Fibroids are the most common benign tumors in women and are typically diagnosed in the middle to later reproductive years. They have been a major indication for hysterectomy in the United States.Reference Buttram and Reiter81 Although most fibroids are asymptomatic, they can result in heavy and painful menstruation, urinary frequency and urgency and pelvic or abdominal discomfort. As with most gynecologic disorders, prevalence varies by study population or sample, and is reported to affect as many as ≈40% of reproductive aged women.Reference Buttram and Reiter81–Reference Wallach and Vlahos83 The cumulative incidence of uterine fibroids increases with age, reaching 70–80% by age 50 years.Reference Viswanathan, Hartmann and McKoy84 Incidence reportedly varies by race/ethnicity with a higher percentage of African-American women affected in comparison with Caucasian women.Reference Baird, Dunson, Hill, Cousins and Schectman85 Of interest is the strikingly higher aromatase mRNA levels in leiomyoma compared with adjacent myometrium in African-American compared with Caucasian women, highlighting the local role of estrogenic activity in the origin of fibroids and the potential role of estrogenic EDCs.Reference Ishikawa, Reierstad and Demura86 Approximately 13% of women undergoing in vitro fertilization have fibroids.Reference Klatsky, Tran, Caughey and Fujimoto87 The impact of uterine fibroids on fecundity is difficult to delineate, given the varying phenotypes, non-uniform diagnostic approaches and selection bias arising from currently available studies.
Researchers have induced uterine leiomyomas in mice by administering DES to the dam on gestational days 9–16,Reference Newbold, Moore and Dixon88 suggesting a possible in utero origin. Similarly, women exposed to DES are reported to have a higher odds of developing fibroids in comparison with unexposed women in a few studies,Reference Baird and Newbold89, Reference D'Aloisio, Baird, Deroo and Sandler90 but not all.Reference Wise, Palmer and Rowlings91 D'Aloisio et al.Reference D'Aloisio, Baird, Deroo and Sandler90 noted that women who were fed soy formula during infancy compared with women fed non-soy formula or breast milk during infancy also had a higher odds of having fibroids than unexposed women.Reference Wise, Palmer and Rowlings91 These latter findings underscore the potential importance of dietary exposures in the context of EDCs during sensitive windows of human development. Although limited, there is some suggestion that EDCs may be associated with fibroids, particularly in the context of other lifestyle factors such as diet.
Polycystic ovarian syndrome (PCOS)
PCOS is a common gynecologic disorder characterized by multiple ovarian cysts and a varied phenotype. The prevalence of PCOS based upon the National Institute of Child Health and Human Development criteria is approximately 7% among women aged 18–45 years, though it is reported to be higher for black (8%) than white (5%) women.Reference Azziz, Woods and Reyna92 Hispanic women are reported to have a prevalence of ≈13%.Reference Goodarzi, Quinones and Azziz93 Using the Rotterdam criteria for PCOS,94 diagnosis requires the presence of two of the following signs: (1) clinical or biochemical evidence of hyper androgenism; (2) intermittent or absent menstrual cycles; and/or (3) polycystic ovary morphology as visualized by ultrasound. Prevalence is reported to be 6.3% in a representative population in Sri Lanka using these criteria, reflecting the addition of polycystic ovary morphology as visualized by ultrasound.Reference Kumarapeli, Seneviratne, Wijeyaratne, Yapa and Dodampahala95
Of all gynecologic disorders, PCOS has the most evidence supporting an in utero etiology. Higher birth weights and longer gestations have been reported for girls who develop PCOS in comparison with unaffected girls suggestive of an in utero origin. Creswell et al.Reference Cresswell, Barker and Osmond96 reported two forms of PCOS reflecting different in utero origins. Thin women with PCOS had altered hypothalamic release of luteinizing hormone possibly resulting from a prolonged gestation, whereas obese hirsute women with polycystic ovaries had higher birth weights and maternal obesity. Wickenheisser et al.Reference Wickenheisser, Nelson-DeGrave and McAllister97 speculated that PCOS was a primary ovarian abnormality leading to androgen excess supporting early work in sheep and rhesus monkeys.Reference Abbott, Barnett, Bruns and Dumesic98 We are unaware of any human studies examining in utero chemical exposures in humans and later onset PCOS. Higher serum BPA concentrations have been reportedReference Takeuchi, Tsutsumi, Ikezuki, Takai and Taketani99 in women with PCOS, as compared with women without PCOS, and in another study, irrespective of obesity.Reference Tsutsumi100 Of added note is the observation that PCOS can be induced in rhesus monkeys and sheep following in utero androgen exposure at environmentally relevant doses during sensitive windows of human development.Reference Abbott, Dumesic and Franks101, Reference Eisner, Barnett, Dumesic and Abbott102 When combined, these data support additional research focusing on EDCs during critical windows and PCOS.
Endometriosis
Endometriosis is a complex disease characterized by the presence of endometrial glands and stroma outside the uterine cavity. Although incidence is difficult to estimate and is highly dependent upon the study population, it has been estimated to be 1.9 per 1000 person-years.Reference Leibson, Good and Hass103 Prevalence varies considerably depending upon the choice of study populations and ranges from 10–15% for the general populationReference Houston104, Reference Olive and Schwartz105 to 20–65% of women seeking care for pain or for infertility services.Reference Carter106–Reference Wardle and Hull110 The diagnosis of endometriosis is reported to be increasing, though reasons remain unknown. Despite a plethora of alleged etiological hypotheses, its origin remains unknown. During the past few decades, interest in an environment etiology has arisen in part following the work of Rier et al.Reference Rier, Martin, Bowman, Dmowski and Becker111 who observed a dose-dependent relationship between dioxin and endometriosis severity in Rhesus monkeys. Subsequently, the authors observed similar effects for select dioxin-like PCB congeners.Reference Rier, Turner and Martin112 Of the 20 published human studies focusing on EDCs and endometriosis, about half reported significant associations with endometriosis, including those with exposure to dioxins,Reference Heilier, Nackers and Verougstraete113, Reference Mayani, Barel, Soback and Almagor114 metals,Reference Jackson, Zullo and Goldberg115 phthalatesReference Cobellis, Latini and De116, Reference Reddy, Rozati and Reddy117 and PCBs.Reference Gerhard and Runnebaum118–Reference Quaranta, Porpora and Mattioli121 It is important to note that many of the negative studies utilized a sampling framework that encompassed a low percentage of women with endometriosis relative to the size of the overall study sample, did not have laparoscopically or histologically confirmed endometriosis or utilized laboratory practices such as automatically substituting values for concentrations below the limits of detection and/or lipid-adjust concentrations. These laboratory practices have been empirically demonstrated to introduce bias.Reference Richardson and Ciampi122–Reference Schisterman, Vexler, Whitcomb and Liu124 To date, we are unaware of any studies with in utero measurement of EDCs and subsequent gynecologic disorders such as endometriosis.
There is evolving evidence supporting an in utero origin of endometriosis for other exposures. Missmer et al.Reference Missmer, Hankinson and Spiegelman125 reported that women with prenatal DES exposure had an 80% higher risk of endometriosis compared with unexposed women. Furthermore, the authors found a significant inverse relationship between birth weight and risk of endometriosis. Hediger et al.Reference Hediger, Hartnett and Louis126 observed that endometriosis may be linked to early intrauterine life environment. Specifically, women with endometriosis were more likely to have a lower body mass index at the time of diagnosis and historically, as measured by self-reported body size at various ages through the time of diagnosis. This finding was recently confirmed in the Nurses’ Health Study, where an inverse association was observed between body size in early childhood and endometriosis risk in adulthood.Reference Vitonis, Baer, Hankinson, Laufer and Missmer127 Buck Louis et al.Reference Buck Louis, Hediger and Pena128 reported a significant reduction in the odds of endometriosis among women whose mothers smoked during their pregnancy compared with non-smoking mothers. The authors suggested that smoking may be associated with a lower estrogenic state.
Reproductive site cancers
The temporal patterns for reproductive site cancers offer a conflicting picture with some rates increasing while others are decreasing. Over the past 25 years, breast cancer incidence rates have risen approximately 30% in westernized countries, possibly as a result of population changes in reproductive behaviors and better screening.Reference Colditz, Sellers and Trapido129 Incidence has recently decreased in the United States, possibly following reductions in the use of hormone replacement therapy in the context of mammography utilization.Reference Garcia, Jemal and Ward130 Overall ovarian cancer rates have decreased in some countries such as the United States from 16 per 100,000 person-years in 1975 to 13 per 100,000 in 2007,131 while they are increasing in other geographic areas. For example, rates tripled between 1974–1985 and 1992–2001 in Sassari, Sardinia (4.27/100,000 person-years v. 11.9/100,000, respectively),Reference Cossu, Budroni and Capobianco132 suggesting considerable geographic variation in temporal patterns.
Many reproductive factors have been associated with various reproductive site cancers, particularly when assessing histological types. For instance, low parity, infertility, early age of menarche and later age reaching menopause are associated with increased risk of epithelial ovarian cancer.Reference Mok, Kwong and Welch133 Interestingly, women with synchronous primary cancers of the endometrium and ovary also have distinct reproductive characteristics including younger age, premenopausal status and nulliparity.Reference Grammatikakis, Zervoudis, Evangelinakis and Tziortzioti134
For the past few decades, considerable research has focused on the role of EDCs and reproductive site cancers with equivocal results as previously reviewed.Reference Ahlborg, Lipworth and Titus-Ernstoff135 Both PCB and DDT congeners have been associated with a small increased risk of endometrial cancer.Reference Sturgeon, Brock and Potischman136, Reference Weiderpass, Adami and Baron137 Adult exposure to EDCs and various female reproductive effects has recently been reviewed and evidence suggests that an important role may be elucidated in the years to come as we design better studies to follow women from conception through adulthood.Reference Crain, Janssen and Edwards138 The extent to which reproductive site cancers may arise in utero remains speculative, but increasingly such cancers are being assessed as possible intermediates in the cancer pathway similar to the TDS paradigm. Concerted research initiatives are fueled, in part, by the carcinogenic role of DES, including transgenerationally.
A recent meta-analysis focusing on in utero exposures and breast cancer reported that birth weight and length were positively associated with increased risk; however, the effects of gestational age and DES on breast cancer risk remain equivocal.Reference Palmer, Wise and Hatch139, Reference Xue and Michels140 Strohsnitter et al.Reference Strohsnitter, Noller and Titus-Ernstoff141 recently reported that in utero exposure to cigarette smoke may reduce the risk of breast cancer later in life by approximately half, perhaps by reducing the estrogenic intrauterine environment for the developing mammary glands.
Fecundity and later onset adult health
In weighing the available literature, a suggestive, although incomplete, body of evidence supports a possible early origin for female fecundity and its implications across the lifespan including fecundity impairments, gynecologic disorders, gravid and later onset adult diseases. Despite limited study, there is increasing speculation that gynecologic disorders may be associated with pregnancy complications, gravid and chronic diseases, more generally. For example, fibroids may disproportionately affect nulliparous women; among parous women, they may be associated with a longer TTP, greater likelihood of pregnancy loss and/or preterm delivery.Reference Somigliana, Vercellini and Daguati142 As yet, a critical data gap is our inability to delineate the causal pathway between fecundity, parity, gravid health and later onset adult diseases.
The relationship between PCOS and gravid health has received considerable attention, given the growing recognition that affected women are at increased risk for preeclampsia, pregnancy-induced hypertension and gestational diabetes in comparison with unaffected women.Reference Boomsma, Eijkemans and Hughes143 In addition, risks for later onset adult diseases such as cardiovascular diseases and metabolic syndrome remain for women with PCOS.Reference Azziz, Woods and Reyna92 Conversely, women with endometriosis are reported to be at reduced risk for preeclampsia and pregnancy-induced hypertension in comparison with unaffected women,Reference Brosens, De and Hamerlynck144 but at greater risk for autoimmune and other endocrine disorders such as hypothyroidism, fibromyalgia, chronic fatigue syndrome, rheumatoid arthritis, systemic lupus erythematosus, Sjogren's syndrome, asthma, allergies and multiple sclerosis.Reference Sinaii, Cleary, Ballweg, Nieman and Stratton145 Recent evidence suggests that women with endometriosis are at higher risk of reproductive site cancers. For example, using published survey results for the general US female population, women with endometriosis had higher rates of ovarian cancer, particularly for women with ovarian endometriomas diagnosed after 50 years of age.Reference Kobayashi, Sumimoto and Moniwa146 Among Swedish women, increased risks were observed for ovarian cancer, non-Hodgkin's lymphoma, endocrine and brain tumors among women with endometriosis in comparison with unaffected women.Reference Brinton, Gridley, Persson, Baron and Bergqvist147 These findings may suggest a shared etiology for endometriosis and ovarian cancers, or the possible transformation of endometriosis into malignant disease.Reference Ogawa, Kaku and Amada148, Reference Yoshikawa, Jimbo and Okada149
Conclusion and critical data gaps
Although incomplete, the ODS hypothesis provides a framework for conceptualizing an early origin for female fecundity and, subsequently, health across the woman's lifespan. The diversity of effects that may be considered in relation to EDCs or other lifestyle factors includes fecundity and fertility endpoints, gynecologic disorders, gravid health and later adult diseases arising from epigenetic changes during critical and sensitive windows of human development. Following exposure, the embryo/fetus undergoes permanent reprogramming or functional changes with possible lifelong and transgenerational implications. To this end, fecundity impairments and gynecologic disorders may be an early signal indicative of programming effects with implications for later onset adult diseases. This avenue of research requires study designs that are capable of identifying and measuring multiple exposures across the continuum of critical and sensitive windows in the context of other influences such as nutrition and lifestyle. This avenue of evolving research is the so-called ‘exposome’ paradigm, and is offered to complement genomic and epigenetic research emphasizing the importance of a genetic/epigenetic-environmental approach to health and disease.Reference Wild150 Life course epidemiological methods coupled with newer statistical models appropriate for the hierarchical data structure and a spectrum of study endpoints offer promise for designing research sensitive to critical data gaps.
Acknowledgment
This study is supported in part with intramural funding from the Eunice Kennedy Shriver National Institute of Child Health & Human Development.
Statement of Interest
None.
