A large Danish study looking at nearly 900,000 births finds no link between birth defects and the use of oral contraceptives around the time of conception and early pregnancy. The global sigh of relief is almost audible.

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A large Danish study finds no increase in the risk of birth defects due to contraceptive pill usage.

The oral contraceptive pill is the most commonly used form of contraception by women in America. An estimated 16% of women aged 15-44 use the contraceptive pill regularly.

The reasons for this prevalence are clear; they are simple to take and, when used accurately, are 99% effective.

However, within the first year of taking the oral contraceptive, around 9% of users become pregnant. This is generally through missed doses, illness or drug interactions. Also, when a woman decides to become pregnant and stops taking the pill, she may become pregnant within just a few menstrual cycles.

In both of these cases, the fetus may have some exposure to the sex hormones in the pill.

Despite the vast numbers of women and fetuses exposed to these hormones, very little is known about their potential effects on the unborn child.

Previous research into fetal effects of contraceptive pills

Research into this matter spans decades, and for good reason. It is known that exogenous sex hormones increase levels of vitamin A and decrease the levels of folate in the mother. Both of these changes are considered to be theoretically teratogenic; in other words, they have the potential to disturb the development of the embryo.

Although the subject has been investigated, much of the findings are tentative at best. Some of the previous studies were reliant on retrospectively self-reporting usage of the contraceptive pill. The concern is that a woman who has a child with a birth defect may recall their oral contraceptive exposure differently.

Other studies utilized only small sample sizes, and because birth defects are rare, unless a huge sample is used, it is difficult to infer a genuine relationship with any degree of accuracy.

Some earlier research found no links between oral contraceptives and birth defects; others found associations between the pill and hypoplastic left heart syndrome (malformed left ventricle), limb defects, gastroschisis (the baby's intestines protrude through a hole in the abdominal wall) and urinary tract anomalies.

A fresh look at the problem

The current study, published in The BMJ, is the largest and most wide-ranging study to delve into the details of this important topic.

A team of Danish and American researchers poured over the records of live births in Denmark between 1997-2011. In total, after discounting children born with birth defects caused by a known factor, for instance, fetal alcohol syndrome, the team was left with 880,694 births.

Of this huge sample of births, 2.5% had birth defects within the first year of life.

From a national registry database, the team was able to pinpoint the last time the mothers were prescribed oral contraceptives.

Of the mothers, 68% used the contraceptive pill but stopped using it more than 3 months before becoming pregnant. In total, 8% continued using the pill within the 3 month window before pregnancy and 1% used the oral contraceptive beyond pregnancy.

The team adjusted for other factors that can influence the likelihood of using contraceptives or potentially increase the risk of birth defects. These included education level, household income, previous pregnancies with birth defects, smoking during pregnancy and other prescription drug use.

The findings were clear-cut and are sure to put expectant mothers' collective minds at ease. The quantities of birth defects were stable across all four groups:

  • Never used oral contraceptives: 25.1 birth defects per 1,000 births
  • Used oral contraceptives more than 3 months before pregnancy: 25.0 birth defects per 1,000 births
  • Oral contraceptive use 0-3 months before pregnancy: 24.9 birth defects per 1,000 births
  • Oral contraceptive use during early pregnancy: 24.8 birth defects per 1,000 births.

The authors of the research say:

"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."

Although the authors are quick to note that this is an observational study and, therefore, cannot claim to prove cause and effect, it certainly adds compelling evidence for the level of safety of the oral contraceptive.

Medical News Today recently covered research showing a reduction in endometrial cancer through contraceptive pill usage.