File photo: Previously, they were told they could enjoy one or two drinks once or twice a week.

Pregnant women can have the occasional drink without harming their baby, researchers claim.

They say official advice to abstain from alcohol is overly cautious and sexist. Women should be left to make their own decisions, according to academics and the abortion provider BPAS.

They claim government guidelines cause needless anxiety, particularly for those who drank heavily before finding out they were pregnant. But senior midwives and doctors’ leaders insist the safest option is to stop drinking, both during pregnancy or when trying to conceive.

Last year Dame Sally Davies, the chief medical officer, issued alcohol guidelines advising pregnant women to abstain completely.

Previously, they were told they could enjoy one or two drinks once or twice a week.

Many other countries have long told women to give up alcohol as soon as they get pregnant.

There is some evidence that having just a couple of glasses of wine a week increases the risk of the baby being born premature, underweight or having a low IQ. But Ellie Lee, of the centre for parenting culture studies at the University of Kent, said the new advice had gone down an "overtly precautionary route".

She claimed evidence that small amounts of alcohol were harmless was overlooked as being "insufficiently robust". But she would not suggest how much pregnant women should be allowed to drink.

Clare Murphy of the British Pregnancy Advisory Service said: "There can be real consequences to overstating evidence, or implying certainty when there isn’t any.

"Doing so can cause women needless anxiety and alarm. It assumes women cannot be trusted to understand risk, and when it comes to alcohol, the difference between low and heavy consumption.

"Women don’t stop being people with the capacity and the right to make their own informed choices just because they are pregnant."

The guidelines will be debated by BPAS and the University of Kent today at a conference at Canterbury Christ Church University. A survey last month showed that at least 28 percent of women drink during pregnancy.

Previous research had put the figure at 40 percent. Just four percent of pregnant women drink in Norway, seven percent in Sweden and 10 percent in Poland.

Britain also has a high rate of foetal alcohol syndrome, with 61.3 cases per 10 000 children compared with a global average of 15.

Janet Fyle of the Royal College of Midwives said: "Our advice has always been clear and unequivocal – if you are thinking of becoming pregnant or are pregnant, then it is best to avoid alcohol."

Professor Lesley Regan, president of the Royal College of Obstetricians and Gynaecologists, said: "There is no proven safe amount of alcohol a woman can drink during pregnancy.

"Heavy drinking can cause foetal alcohol spectrum disorders and it has also been linked with an increased risk of miscarriage. Abstinence from alcohol is the safest option, in particular for women trying to conceive or during the first three months of pregnancy."