It is the underlying hypertension of the mother, rather than the use of antihypertensive drugs in early pregnancy that increases the risk of birth defects, research findings show.

The new study led by Dr De-Kun Li of the Kaiser Foundation Research Institute in California, set out to see if there was an association between using ACE inhibitors during a woman's first trimester and birth defects. Angiotensin-converting enzyme (ACE) inhibitors are a type of antihypertensive medication commonly prescribed to tackle hypertension.

ACE are known to have a toxic effect on fetuses in the second or third trimesters, but their effects on a fetus during the mother's first trimester are still unclear.

The study showed that women who used ACE inhibitors in their first trimester were more likely to have a baby with some form of birth defect compared with women who did not have hypertension or who had not used any form of antihypertensive medication.

However, a similar elevated risk was found among women who used other antihypertensive drugs and those with hypertension who did not take any antihypertensive medication.

"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," the researchers conclude.

The new research was published online in the British Medical Journal.

"It would appear reasonable to conclude that first-trimester exposure to ACE inhibitors poses no greater risk of birth defects than other antihypertensives, and that it is the underlying hypertension that places the fetus at risk," says Professor Allen Mitchell of Boston University in the accompanying editorial.