The Pill Doesn’t Cause Birth Defects

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A new study suggests taking oral contraceptives shortly before or during a pregnancy won't cause birth defects. UCI UC Irvine/Flickr

New research seeks to put to rest the possibility that oral contraceptive use leads to birth defects.

In an analysis of data on more than 800,000 infants, researchers at Harvard University’s T.H. Chan School of Public Health didn’t find a statistical difference in rates of birth defects in women who had a history of taking oral contraceptives before or during pregnancy versus those who did not. In both groups (and accounting for stillbirths and abortions) birth defects occurred in 25 per 1,000 infants.

The researchers say exposure to the pill won’t likely cause birth defects, regardless of whether a woman accidentally becomes pregnant while on the pill (due to incorrect or inconsistent use) or conceives soon after stopping it.

The study, published Wednesday in BMJ, analyzed data from several health registries out of Denmark that span from 1997 through 2011. This data set is unique because every patient in the country is given an identification number to track the health care services received, including any procedures, doctor’s appointments and pharmacy visits. It accounted for 880,694 infants and was based on one-year follow-ups. The researchers estimated oral contraceptives use by looking at the dates each mother last filled their prescription.

Oral contraceptives are 99 percent effective at preventing pregnancy—but only when used correctly. Approximately 10 percent of women who take the pill accidentally become pregnant within the first year of using it. This can occur if a woman inadvertently misses a pill or takes the wrong one during her monthly cycle. According to the Guttmacher Institute, at least 40 percent of women who use birth control pills do so inconsistently.

Approximately one-fifth of the women in the BMJ study had never used oral contraceptives before conceiving, while two-thirds of the women stopped using the prescription three months or longer before their pregnancy. Eight percent had discontinued use within three months of becoming pregnant, and 1 percent—10,000 women—were using oral contraceptives while pregnant.

The synthetic female sex hormones in the pill—either estrogen and progestin or progestin alone—prevent a woman from ovulating and causes the cervical mucus to thicken, inhibiting the sperm’s ability to fertilize an egg. But there is little research on the effects these man-made hormones will have in utero, or on how long they remain in the system once a woman goes off the pill. Most previous studies on the subject were conducted decades ago and based on self-reported data, which is often unreliable.

One study published in 1985 in Canadian Family Physicians found oral contraceptives may deplete levels of vitamin B12 and folate, and that these levels don’t return to normal for at least three months after a woman discontinues use of the drug. Folate (or folic acid) and B12 are critical to fetal development. In pregnancy, folic acid—a type of B vitamin—helps prevent defects in the neural tube, the part of the fetus that will eventually become the brain and spinal cord.

Other studies conducted on the topic have looked at contraceptive use and the risk for certain complications such defects of the heart or limbs or urinary tract abnormalities. But in their analysis, the Harvard researchers didn’t find any confirmation of any such risk.