Pregnancy photoOral contraceptive use just before or during pregnancy is not associated with an increased risk of major birth defects, according to a joint US and Danish study.

The large prospective observational study examined the association between oral contraceptive use around the time of conception, and into pregnancy, with major birth defects. Findings from the study revealed no increased risk of any major birth defect associated with oral contraceptive exposure.

Around 9% of women become pregnant in the first year of use because of missing a dose, taking the pill with certain other medications, or illness. All live births, birth defects, and maternal medical conditions were analysed from several national Danish medical registries between 1997 and 2011.

The final analyses included 880,694 live born infants, 2.5% of whom had a major birth defect within the first year of life excluding those defects with known causes, such as fetal alcohol syndrome or chromosomal aberrations.

Overall, 8% recently stopped using oral contraceptives within 0 to three months before pregnancy, and 1% used oral contraceptives beyond pregnancy – both categories were analysed as exposed.

Study lead Dr Brittany Charlton said: "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."

The prevalence of major birth defects, per 1000 births, was consistent across each group: 25.1 for never users, 25.0 for oral contraceptive use more than three months before pregnancy, 24.9 for oral contraceptive use within 0 to three months before pregnancy, and 24.8 for oral contraceptive use after pregnancy.

Adjustments were made for other risk factors for birth defects and those that increase the likelihood of contraceptive use.

These included age at pregnancy, place of birth, level of education, household income, history of birth defects from previous pregnancies, smoking in pregnancy, and healthcare use, such as prescription drug use and hospital admissions.

Although it is an observational study, so no definitive conclusions can be made about cause and effect, nevertheless, the authors say the findings should reassure women who have a breakthrough pregnancy during oral contraceptive use.