Pregnant women worry about all kinds of things. Can I drink alcohol? (No.) Can I take antidepressants? (Maybe.) Can I do the downward dog? (Yes.)
Now there's one less thing to fret about: harm to the baby when the mother takes birth control pill right before conceiving, or during the first few months of pregnancy.
According to a study covering more than 880,000 births in Denmark, the overall rate of birth defects was consistent for women who had never taken the pill at all, for those who had used it before getting pregnant and for those who continued on the pill in early pregnancy. (There were about 25 birth defects per 1,000 births for all groups.)
The study is important because so many women take the pill – about 16 percent of women of childbearing age in the U.S. When used perfectly, the failure rate of the pill is less than 1 percent, but that jumps to 9 percent under typical use because of missed pills, drug interactions or illness. That means a lot of embryos are exposed to the hormones used in the pill, which can linger for a few months after a woman stops taking it.
"Our findings are really reassuring," says Brittany Charlton, an author of the study and a researcher in the Harvard T.H. Chan School of Public Health's epidemiology department. The results also confirm most of the previous research, which has pointed to no overall increase in major birth defects, she says. This study, published in the medical journal BMJ, used national birth, patient and prescription registry data to track contraceptive prescriptions among women who gave birth, then looked at whether birth defects were associated with pill use.
Many earlier studies started with a group of women whose children had birth defects, then looked backward to ask them about previous contraceptive use, says Charlton. That can be troublesome because parents of babies with birth defects may be more likely to recall the medications they took around the time of pregnancy than parents of kids with no birth defects.
Dr. Diane Horvath-Cosper, a fellow of the American College of Obstetricians and Gynecologists, says she's been telling concerned pregnant patients that while studying this topic is difficult, the existing research hasn't raised concerns. This study, she says, "provides me with a lot more convincing evidence that there's no link between oral contraceptives and birth defects."
She also says it may encourage physicians to tell women to start new pill prescriptions immediately, rather than waiting out the month to make sure they're not pregnant. Starting quickly will help prevent accidental pregnancies, and if there is a positive pregnancy test, "you're not going to hurt the baby," she says.
The study didn't find any significantly increased risk associated with any subgroup of birth defects. But the authors said because of small sample sizes, they couldn't rule out some association with two specific defects. Their data showed a higher risk of one rare type – hypoplastic left heart syndrome – among women who used the pill while in early pregnancy, but there were only three cases and the results might well have occurred by chance.
Kim Waller, an epidemiologist at The University of Texas Health Science Center at Houston School of Public Health, was an author of a 2010 study looking at oral contraceptive use and 32 categories of birth defects. Her study found no association between the pill and most kinds of birth defects, but did see a statistically significant risk of the same heart problem among babies born to women who used the pill during the first trimester. She says given the results of both studies, she's interested in studying pill use and that particular defect.
The latest study didn't distinguish between different types of oral contraceptives, and that may be worth researching in the future, the authors said.
Katherine Hobson is a freelance health and science writer based in Brooklyn, N.Y. She's on Twitter: @katherinehobson.