Most miscarriages probably occur in the first three weeks after conception,
when the placenta is developing, but because women seldom even know they are
pregnant during this time, it is hard to study. Most knowledge of pregnancy loss
relates to what happens after about the sixth week.
Pablo Nepomnaschy at the National Institute of Environmental Health Sciences
in North Carolina, US, and his colleagues followed 61 women for a year. Most of
the women were in their early twenties and all had at least one previous child,
which they were still breastfeeding. The women were poor, did not get enough to
eat and were often ill.
The women collected urine samples first thing in the morning three times a
week, which was examined for evidence of pregnancy and for cortisol levels a
measure of stress. During the study, there were 22 pregnancies, nine of which
were carried to term.
Evolutionary mechanism
The researchers found that women whose cortisol levels significantly
increased over their baseline during the first three weeks of pregnancy were 2.7
times more likely to suffer a pregnancy loss than women with no cortisol
increase. Nine of 10 pregnancies to women with high cortisol levels ended in
miscarriage, compared with only four of 12 to women with stable cortisol.
Theres a lot of controversy over whether stress can cause early
pregnancy loss, says Nepomnaschy, who is now following up with a larger
study.
The researchers speculate that, from an evolutionary point of view, an early
miscarriage when the mother is stressed may help to conserve maternal resources
until external conditions improve, increasing the chances carrying a later baby
to term.
Journal reference: Proceedings of the National Academy of Sciences (DOI:
10.1073/pnas.0511183103)
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