Studies in mice indicate that the adverse reproductive tract outcomes associated with prenatal DES exposure may be mediated by alterations in the expression of genes involved in estrogen signaling/regulation or patterning of the reproductive tract (McLachlan et al., 2001
; Nelson et al., 1994
). Mouse studies also suggest that epigenetic alterations may be transmitted to the next generation, although the effects in the prenatally exposed females may differ from those in their daughters. For example, an excess of uterine adenocarcinoma is seen in the prenatally exposed mice and in their daughters, but infertility affects only the prenatally exposed females (Newbold, et al., 1998
In this study, based on mothers’ reports, the overall proportion of offspring affected by birth defects (3.7%) resembled rates reported for US whites (3.5%) (Texas Birth Defects Registry, 2005
). Although our data suggested that birth defects may be elevated in the daughters of prenatally DES-exposed women, we did not observe a pattern of defects consistent with those reported in the prenatally exposed offspring. For example, in the third generation sons, we found limited support for an association between the mothers’ prenatal exposure and penile/testicular defects, which may affect the prenatally exposed men (Bibbo et al., 1977
; Coscrove et al., 1977
; Gill et al., 1979
; Wilcox et al., 1995
). Although the present study suggested a possible excess of cryptorchidism, this finding was compatible with chance. None of the mothers reported sons affected by epididymal cysts, which may affect men with prenatal DES exposure (Bibbo et al., 1977
; Coscrove et al., 1977
; Gill et al., 1979
; Wilcox et al., 1995
). An excess of hypospadias has been reported for sons of prenatally exposed women (Klip, et al., 2001
), but DES exposure was unverified in that study, and the association was unconvincing in the NCI combined cohort study (OR: 1.7; 95% CI: 0.4, 6.8) (Palmer, et al., 2005
None of the mothers reported daughters affected by reproductive tract anomalies, including T-shaped uterus and abnormalities of the cervix, outcomes associated with prenatal DES exposure in women (Shapiro & Slone, 1979
). However, for the most part, such conditions would become evident only when the daughter underwent a work-up for infertility or reproductive dysfunction. Less than half of the women in the third generation study had reached age 18 as of 1997, when the mothers were asked to report birth defects in their offspring; consequently, the study population may have been too young to manifest such outcomes, even if an association existed. A clinical study of the adult daughters of prenatally DES-exposed women did not identify gynecological anomalies (Kaufman & Adam, 2002
), but the sample was small (n = 28) and certain conditions, such as structural abnormalities of the uterus or fallopian tubes, might not be evident on physical examination. Further study of the third generation will be needed to determine whether the reproductive tract toxicity observed in prenatally DES-exposed women also affects their daughters.
Based on the mothers’ reports, our data suggested an association between the mothers’ prenatal DES exposure and heart conditions in the daughters. However, this association may be an artifact of underreporting of these conditions by the unexposed mothers. Population-based birth defects registries indicate that rates of cardiac defects are similar for males and females (Texas Birth Defects Registry, 2005
). In our data, the proportions affected were similar for the exposed sons (0.7%), unexposed sons (0.6%) and exposed daughters (0.8%). The proportion was markedly reduced, however, in the unexposed daughters (0.2%), consistent with mothers’ under-reporting of heart defects in this group. There are no previous reports of cardiac defects in the offspring of women who were exposed prenatally to hormones or specifically to DES. An early cohort study (Heinonen, et al., 1977
) and a case-control study based on birth certificate data (Janerich, et al., 1977
) suggested a two-fold increase of congenital heart disease in individuals who were prenatally exposed to exogenous female hormones (any type), but the findings were not corroborated by a meta-analysis of prospective studies assessing the effects of prenatal exposure to oral contraceptives (Bracken, 1990
). A syndrome involving vertebral anomalies, anal atresia, cardiac defects, TEF, renal anomalies, and limb reduction (VACTERL) has been postulated in relation to prenatal exposure to estrogen/progestogen (Nora & Nora, 1975
), but evidence supporting the association is mixed (Shapiro & Slone, 1979
). A recent study of third generation offspring showed an increased risk of TEF in third generation offspring (Felix, et al., 2007
), but this association was not clear in our data.
In this study, the higher prevalence of defects in the exposed offspring, based on the mothers’ data, may reflect reporting bias. Participation bias is less likely, as participation was good among the mothers and the question on birth defects was not included in the first combined cohort questionnaire. The overall percent of defects was higher in the daughters’ self-reported data than in the mothers’ data, which might reflect more thorough reporting by the daughters, or perhaps a tendency of mothers to grant permission to enroll daughters who had birth defects. The proportions of exposed and unexposed women affected by birth defects were more similar in the daughters’ self-reported data than in the mothers’ data, possibly because reporting bias was minimized in the daughters’ data. Twenty-eight percent of the daughters of DES-exposed women were unaware of their exposure, and 60% of the daughters of unexposed women either were not sure of their mothers’ exposure or believed their mothers were DES-exposed (Titus-Ernstoff et al., 2006
). Finally, prenatal DES exposure in women is associated with infertility and adverse pregnancy outcomes, whereas all of the women who contributed daughters to the Third Generation Study and most of the women reporting on their offspring had at least one live birth. If these women were less impacted by prenatal DES exposure, any potential association between DES and conditions in the next generation might be attenuated.
We found poor agreement between the mothers’ and study participants’ reports of birth defects, primarily due to fewer reports by the mothers, who omitted even severe birth defects reported by the daughters. Possibly, mothers may have been less likely than daughters to report conditions that were not apparent at the time of birth. Anomalies such as a missing forearm, club foot, and cleft palate, which would have been immediately evident, were among those conditions reported by both the mother and the daughter. In contrast, more than half of daughters’ reports of hip anomalies, which might become apparent weeks or months following delivery, were not replicated by the mother. Nevertheless, in the absence of a gold standard, we cannot say for certain whether birth defects were under-reported by the mothers or over-reported by the daughters.
In conclusion, our data raise the possibility that the offspring of prenatally DES-exposed women may have an increased frequency of birth defects. We did not observe a pattern of defects resembling those observed in the prenatally exposed men or women, although reproductive tract changes in third generation women might not become evident until childbearing ages. We cannot exclude the possibility that our findings were distorted by bias. In particular, the excess of cardiac defects in the exposed daughters may reflect an underreporting of such conditions by unexposed mothers rather than a true excess in the daughters of the exposed.