Oral contraceptive exposure just before or during pregnancy is not associated with an increased risk of major birth defects, according to results of a Danish study.
“This study is further evidence of exposure to oral contraceptives not being associated with an increased risk of major birth defects,” says Natika Halil, chief executive of UK sexual health charity FPA, which welcomes the research. “Previous research had also shown there to be no consistent findings of specific fetal abnormalities,” she adds.
Although several studies have showed no risk associated with oral contraceptive use before or during pregnancy and the health of the offspring, the link has been unclear for some years, the researchers say. In particular, it was unclear how long the effects of the circulating sex hormones released by the pills might last.
Exogenous sex hormones have been shown to increase plasma concentrations of vitamin A, which is known to cause birth defects. Studies also suggest that serum levels of the B vitamin folate are reduced after oral contraceptive use and remain reduced for up to three months after discontinuation, which could also lead to a range of birth defects.
The researchers, led by Brittany Charlton of the Harvard TH Chan School of Public Health in Boston, Massachusetts, ran a nationwide study in Denmark to investigate whether oral contraceptive use less than three months before pregnancy was associated with an increased risk of major birth defects. Data were collected from 880,694 infants, 2.5% of whom had a major birth defect, such as an orofacial cleft or limb defect, within the first year of life.
The team found no increased risk of major birth defects associated with oral contraceptive use in the months just before or after the onset of pregnancy. The prevalence of major birth defects (per 1,000 births) was consistent across each of the oral contraceptive exposure groups: 25.1 for mothers who had never used oral contraceptives; 25.0 for mothers who had only used oral contraceptives more than three months before pregnancy onset; 24.9 for mothers who used oral contraceptives less than three months before pregnancy onset; and 24.8 for those who had continued to use oral contraceptives after pregnancy onset.
There was also no increase in the prevalence of any birth defect subgroup (for example, limb defects), although the authors warn that the rarity of birth defects made disaggregation of the results difficult.
“For women who have a breakthrough pregnancy during oral contraceptive use, or even intentionally become pregnant within a few months of stopping oral contraceptive use, any exposure is unlikely to cause her fetus to develop a major birth defect,” say the researchers, whose results were published in the BMJ
on 7 January 2016.
Oral contraceptives are known to be more than 99% effective when the correct regimen is followed, but around 9% of women become pregnant in the first year of use because either they have missed a dose, taken the pill with other medications, or have been ill.
“In reality, inadvertent exposure to contraceptive hormones can be quite common, especially when there is a delay in a pregnancy being confirmed. For example, if amenorrhoea [abnormal absence of menstruation] is assumed to be due to the contraceptive method, or if bleeding is mistaken for a period,” says the FPA’s Halil.
The findings make an important addition to patient care in this area, she adds. “Health professionals can offer further reassurance to women who may worry about potential effects during pregnancy.”
 Charlton BM, MÃ¸lgaard-Nielsen D, SvanstrÃ¶m H et al. Maternal use of oral contraceptives and risk of birth defects in Denmark: prospective, nationwide cohort study. BMJ 2016;352:h6712. doi:10.1136/bmj.h6712