Babies can suffer from foetal alcohol syndrome, mental retardation, development and behavioural abnormalities, and low birth weight, if they are exposed to alcohol in the womb.

The authors said women were facing a "contradictory, confusing barrage of mixed messages” about how to approach pregnancy.

In the UK, the Department of Health recommends that women should avoid alcohol altogether. But, it says, if they do opt to have a drink, they should have no more than one or two units of alcohol (equivalent to one or two small glasses of wine) once or twice a week.

The National Institute for Health and Care Excellence (Nice) advises women to avoid alcohol in the first three months in particular, because of the increased risk of miscarriage.

Meanwhile, the Royal College of Obstetricians and Gynaecologists (RCOG) says drinking too much can harm an unborn baby and women should not drink in the first three months.

But it says that after then drinking one or two units, no more than once or twice a week does not appear to be harmful.

In the BMJ article, the doctors said how and when foetal damage occurs is unknown and will vary according to each individual pregnancy. They add: "Pregnant women must know there is no evidence of a threshold level of alcohol consumption in pregnancy below which there can be certainty that exposure is safe."

Current guidance "flies in the face of evidence", while other countries, including the US, Canada, Denmark, France, Norway, Israel, Mexico, Australia, Ireland, New Zealand and Spain recommend no alcohol in pregnancy, they said.

The number of women who have stopped drinking any alcohol in pregnancy in rising. Latest figures show 48 per cent of women who normally drank alcohol gave it up, compared with 33 per cent in 2005.

The remainder cut down, apart from 2 per cent who reported no change or drinking more.

Dr Patrick O'Brien, a consultant in obstetrics and gynaecology said women were intelligent enough to take on board a range of views.

"We all deal with uncertainty in our lives on a daily basis; pregnant women are no less capable of doing so,” he wrote in the BMJ.

Dr O’ Brien said the evidence was not clear cut, and that going too far in the advice handed out could cause women to lose faith in their doctors.

"If they perceive that we have been making value judgements on their behalf, or professing certainty where none exists, we are certain to lose their trust."

Last year the British Medical Association said pregnent women should be told not to drink alcohol at all.