Women who take female sex hormones while pregnant are twice as likely as other women to have babies with heart defects, according to the largest study ever made of use of these drugs during pregnancy.

The finding will be published in this week's issue of the New England Journal of Medicine, out today.

The women concerned fall into two groups - those who take birth control pills before they know that they're pregnant and, probably the largest group, those whose doctors give them the hormones to try to prevent miscarriages and other problems.

The Food and Drug Administration two years ago warned doctors that use of such drugs "for any purpose" in early pregnancy is "ineffective" and "may seriously damage the fetus."

But the drugs are still being used for this purpose in 1 to 2 per cent of all pregnancies. The Washington Post was told yesterday by a major medical market research service, IMS America Ltd.

This would mean that between 32,000 and 64,000 women a year are still taking them for this reason and giving birth to between 582 and 1,164 babies with heart defects.

The drugs are of two classes - estrogens and progesterones. Both are used in oral contraceptives and other medical compounds.

Doctors in Boston studied 50,282 women who became pregnant between 1958 and 1965. Of these, 1,042 took the hormones for various reasons during the first four months of pregnancy - and had 19 children with birth de-defects, a rate of nearly two such births in every 100 pregnancies.

All the mother's surviving children were followed for up to four to eight years. Ten per cent of those with heart defects died before age 5.

There have been previous studies in far smaller numbers of women that also pointed to the hormones as possible pointed to the hormones as possible instigators of birth defects. But these results were called inconclusive since the numbers of women and babies were so small.

Even the authors of the new study - Drs. Olli Heinonen, Dennis Slone, Richard Monson, Ernest Hook and Samuel Shapiro - said more study is needed to confirm their evidence that these drugs cause defects.

Still, the results mesh with previous observations. "What we have is a situation where thre is no evidence at all that the drugs do any good during pregnancy, and strong evidence now that they may cause harm," said one of the investigators.

Use of the drugs to try to treat problems of pregnancy has dropped by five- to six-fold in recent years, it was estimated yesterday. It was learned in 1971 that one to them - DES or diethylstilbestrol - can cause vaginal cancer in female offspring years later, and the wide dissemination of this knowledge helped cause a sharp drop in hormones' use.

New pregnancy tests have supplanted old ones that used hormones.