Cohn, BA, PM Cirillo, MS Wolff, PJ Schwing, RD Cohen, RI Sholtz, A Ferrara, RE Christianson, BJ van den Berg and PK Siiteri. 2003. DDT and DDE exposure in mothers and time to pregnancy in daughters. The Lancet 361: 2205–06.

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Research by Cohn et al. reveals an unexpected association between DDT and delays in pregnancy in the daughters of exposed women, 30 years after birth. This is the first scientific report ever of a link between DDT and reproductive outcome in women exposed to the contaminant in the womb. Their statistical assessment indicates that the association is unlikely to be a result of chance.

What did they do? Cohn's research team analyzed 289 maternal blood samples that had been taken from women shortly after giving birth to a daughter between 1960 and 1963 and stored in freezers since being drawn. These blood samples were matched with information about the fertility of the women’s daughters, obtained 28-31 years later.

The daughters (28-31 yrs old) provided information via a questionnaire about their fertility history during months when they and their partners were not using contraception. The scientists used this information to determine how long it takes a woman to become pregnant when she is not protected by contraceptives. Women with longer “times to pregnancy” are less likely to become pregnant in any given menstrual cycle. As “time to pregnancy” increases, the chance of subfertility or infertility becomes greater.

The blood samples were analyzed for DDT and its metabolite, DDE. All samples had measurable levels of both contaminants, which is not surprising because the samples were taken before DDT was banned.

What did they find? Cohn’s statistical analysis reveals that the daughters of mothers with higher DDT levels took longer to become pregnant. According to Dr. Cohn, an increase of as little as 10 micrograms per liter was associated with a one-third decrease in the chance of becoming pregnant within a menstrual cycle.

The scientists also discovered that higher levels of DDE, the metabolic breakdown product of DDT, are associated with a smaller but opposite effect. Daughters born to mothers with greater amounts of DDE took less time to become pregnant, controlling statistically for DDT level. For every increase of 10 micrograms per liter, chance of pregnancy increased by one-sixth, about one half of the adverse effect of DDT. There are several possible explanations for this observation. One is that DDE has a protective effect, counteracting the DDT. Another is that women who’s body chemistry converts DDT to DDE more rapidly are less sensitive to DDT’s effect. Still another is that DDE can block the harmful effect of the hormone, androgen, on the ovary, during fetal life or just after birth.

What does it mean? We are still learning the consequences for people of the DDT experiment that played out in the US between the 1950s and the 1970s, after which the contaminant was banned. Strong hints that there might be more yet to learn surfaced two years ago, when CDC scientists reported a striking association between DDT and the likelihood of preterm birth. There, too, the study was based on stored blood samples, analyzed using modern quantitative chemistry and tied to birth records. This work by Cohn et al. suggests additional work with stored blood samples, modern chemistry and health records should prove fruitful. As scientists plan on establishing new cohorts to track from birth through adulthood, they should take full advantage of opportunities that already exist for examining the adult consequences of fetal exposures.

Coincidentally, this study appears one day after a story in the New York Times about the fact that birth rates have fallen to a record low in the United States, 30 years after the heyday of DDT use. The decline in US birth rates has been underway for some time, attributed by classic demographers to changing economic role of women and the increased survivorship of infants. One European review, written in 2002, has proposed that male infertility linked to environmental exposures might be contributing to declining fertility rates. This work by Cohn et al. raises a parallel question about female infertility.