Women who get pregnant within a few weeks of taking birth control pills seem much more likely to have low-weight or premature babies, a new Canadian study indicates. The study does not definitively prove a cause and effect relationship and needs to be confirmed by more research, says lead investigator Xi-Kuan Chen, an epidemiologist at the University of Ottawa. However, babies with low birth weight or who are born prematurely are a growing problem and the cause is not always clear.

The researchers used comprehensive Canadian databases to distinguish three groups of women using oral contraceptives: those who had taken them within 30 days, 31-60 days, and 61-90 days of their last period before getting pregnant -- 1500 in all. They were compared to 6100 women who had not used birth control pills for at least a year before they gave birth.

Outcomes were similar for most of the contraceptive users and non-users. But those who had taken the pill within 30 days of getting pregnant were more than three times as likely to have a very low-weight newborn, and almost twice as likely to have a very low-weight baby. They were also twice as apt to have a baby born at least six weeks premature.

The researchers say their’s is the first study to isolate mothers who had used the pill close to the time they became pregnant, but they concede that other factors may be at play in the birth effects observed. Smoking and obesity are also linked with low birth weight, and the 30-day group of pill users may have included a disproportionate number of unplanned pregnancies, a factor which might, in itself, have affected the outcome. ~ National Post, March 9