Women who accidentally keep taking oral contraceptives after becoming pregnant shouldn't worry about birth defects, a new study suggests.

An analysis of nearly 900,000 births found no connection between oral contraceptive use right before or during pregnancy and birth defects in babies, researchers report in The BMJ.

"For women who do become pregnant either soon after stopping oral contraceptive use or even if they're still taking the pill, they should know that exposure is unlikely to cause their fetus to develop a birth defect," said lead author Brittany Charlton, of the Harvard T.H. Chan School of Public Health in Boston.

Nine percent of women become pregnant in the first year of taking oral contraceptives due to missed doses, drug interactions or illnesses. Other women may get pregnant soon after they stop taking birth control pills, or they might continue on the pills, not realizing they're pregnant.

In these scenarios, the researchers say, the fetus might be exposed to the hormones in the pills. Some research has suggested that exposure may cause birth defects by altering levels of vitamin A and folic acid, which are crucial to fetal development.

"The literature has just been unclear," Charlton told Reuters Health.

For the new study, she and her co-authors used Danish registry data on 880,694 births between 1997 and 2011 to analyze the effects of oral contraceptive use.

During that time, 2.5 percent of children were born with a major birth defect like limb problems, cleft lips or palates, spina bifida or heart defects.

Based on information from a national prescription registry, 8 percent of mothers had stopped using oral contraceptives within the three months before becoming pregnant. Another 1 percent had used contraceptives after becoming pregnant.

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These women had no increased risk of giving birth to a child with a major birth defect, compared to women who never took the drugs or stopped them months before becoming pregnant.

The risk was about 25 birth defects per 1,000 births regardless of exposure to oral contraceptives.

There was also no increased risk of birth defects when the researchers included other factors like stillbirths and induced abortions.

"That's especially reassuring given that our results remained consistent," said Charlton.

She cautioned that the study has some limitations. For example, some birth defects are so rare that even with data on nearly 900,000 births, they still couldn't test for associations with certain defects.

Still, the researchers conclude that exposure to oral contraceptives near the start of a pregnancy is unlikely to cause major birth defects.

"Our finding should reassure women as well as their healthcare providers," said Charlton.