Boston—Oral contraceptives are more than 99% effective with perfect use, but nearly 10% of women become pregnant within their first year of use. That raises concerns about how the pills affect the health of the fetus, according to a new study.
The report, published recently in the British Medical Journal (BMJ), says those worries are generally unwarranted. A study team led by researchers from Harvard Medical School and Boston Children’s Hospital finds that the prevalence of major birth defects was consistent—about 25 per 1,000 live births—across all pregnant women in the study population, regardless of contraceptive use.
“Women who become pregnant either soon after stopping oral contraceptives, or even while taking them, should know that this exposure is unlikely to cause their fetus to develop a birth defect,” explained first author Brittany Charlton, MSc, ScD in a Harvard T. H. Chan School of Public Health press release. “This should reassure women as well as their health care providers.”
Researchers didn’t have to rely on recollections of oral contraceptive use, as in past studies, but instead used data collected from multiple Danish health registries between 1997 and 2011—linked by the unique personal identification number assigned to all residents of Denmark. With a focus on 880,694 live-born infants and their health after a year, the investigators estimated oral contraceptive use based on the date of the mother's most recently filled prescription.
Among the women in the study population, a fifth had never used oral contraceptives before becoming pregnant, and more than two-thirds had stopped using oral contraceptives at least 3 months before conceiving.
Another 8% had discontinued use within 3 months of becoming pregnant, and 1%, or more than 10,000 women, had used oral contraceptives after becoming pregnant. Yet, the prevalence of birth defects was consistent across each category of oral contraceptive use, and remained so even when the researchers added in pregnancies that ended as stillbirths or induced abortions.
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