Hypertension, rather than the use of antihypertensive drugs in early stage of pregnancy, increases the risk of birth defects.

Angiotensin-converting enzyme (ACE) inhibitors are a type of medication, which help relax blood vessels and are commonly prescribed for hypertension. ACE inhibitors are known to have toxic effect on fetuses in the second and third trimesters, but first trimester effects were unknown.

Dr. De-Kun Li and his team at the Kaiser Foundation Research Institute in California studied the link between ACE inhibitors during a woman's first trimester and birth defects.

The researchers analyzed the data of 465,754 mother-infant patients from Kaiser Permanente Northern California region between 1995 and 2008. The information in the data showed which medication was prescribed and dispensed to these women.

Researchers found that hypertension not the ace inhibitor causes birth defects during first trimester of pregnancy, the study results were published Wednesday on the British Medical Journal (BMJ) website. 

"Our finding suggests that it is likely the underlying hypertension rather than use of antihypertensive drugs in the first trimester that increases the risk of birth defects in offspring,” Li said.

In an accompanying editorial, Professor Allen Mitchell from Boston University pointed out that while clinicians must certainly identify and control hypertension, particularly in pregnancy, "we have much to learn about how hypertension can cause birth defects."