Drinking in pregnancy is common, according to a study which labels it a “significant public health concern” even though most women appear to give up once they know they are pregnant.

The authors of the study, in the journal BMJ Open, say the guidelines on safe drinking are confusing, pointing out that Ireland, New Zealand and Australia recommend no alcohol for the entire pregnancy, while the UK says one to two units once or twice a week will do no harm after the first three months.

Substantial numbers of women appear to be taking no heed of guidelines when they become pregnant, according to the findings. The research pulled together results from several studies carried out in different ways in the UK, Ireland, Australia and New Zealand. They found that as much as 80% of more than 17,000 women may have drank during the first trimester of pregnancy.

“It is possible that some of these women were drinking before they knew they were pregnant and that could be contributing to the high rates,” said Dr Linda O’Keeffe of the cardiovascular epidemiology unit at Cambridge University. But, she said, women of childbearing age should be given better information about the risks of alcohol to the foetus.

Most guidance says women should abstain from alcohol if they are, or hope to become, pregnant, she pointed out, adding that they were not blaming women. “That is not what we are trying to do here,” she said.

The different studies also had different results for the amount of alcohol women drink. The highest rates of drinking were in Ireland, where 90% of women said they drank before pregnancy and 82% afterwards. The women who were questioned also admitted to the highest rates of binge drinking – 59% before they were pregnant and 45% during pregnancy. However, other studies showed far lower rates of drinking in Ireland – from 20-46%, with only 3% of women saying they binge drink.

In all countries, drinking dropped dramatically in the second trimester (between three and six months pregnant).

A higher level of education, having other children, and being overweight/obese were also associated with a lower risk of drinking while pregnant, but being a smoker made it more likely that a woman would drink while pregnant.

The authors write that alcohol exposure may occur in over 75% of pregnancies in the UK and Ireland. Since women drink in the first three months, when the development of the foetus is less well understood, they say, “the widespread consumption of even low levels of alcohol during pregnancy is a significant public health concern”.

But the British Pregnancy Advisory Service (Bpas) said this was overstated. “The prevalence of unplanned pregnancy in this country means many women may have an episode of binge drinking before they realise they are pregnant, but the monumental drop in reported binges by the second trimester suggests women alter their behaviour very quickly,” said Clare Murphy, director of external affairs.

“Bpas regularly sees women so concerned about the damage caused by an episode of binge drinking before they recognised they were pregnant they consider ending what would otherwise be a wanted pregnancy. We would like to see greater reassurance to these women that they are extremely unlikely to have caused their baby harm.”

They said they were concerned by a suggestion from the authors that tests to measure women’s alcohol intake in pregnancy needed to be developed. “We would be extremely wary of any measures which sought to further police pregnant women’s behaviour, and undermine the important relationship between a pregnant woman and her healthcare provider,” said Murphy.