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.
Lancet 361: 2205–06.
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.
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
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.
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.
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.
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.
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.
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.