Drinking alcohol while pregnant is common, ranging from 20% to 80% among those questioned in the UK, Ireland, Australia and New Zealand, reveals a study of almost 18,000 women published in the online journal BMJ Open.

Women across all social strata drank during pregnancy, the findings showed. But expectant mums were significantly more likely to be drinkers if they were also smokers.

The researchers base their findings on an analysis of data from three studies: The Growing up in Ireland (GUI) study; the Screening for Pregnancy Endpoints (SCOPE) study; and the Pregnancy Risk Assessment Monitoring System (PRAMS).

The studies variously assessed the amount and type of alcohol drunk before and during pregnancy and involved 17,244 women who delivered live babies in the UK, Ireland, Australia and New Zealand.

The researchers mined the content to gauge the prevalence of, and the factors associated with, drinking alcohol during pregnancy.

Their analysis indicated a high prevalence of drinking, including binge drinking, among mums to be. The prevalence of drinking alcohol ranged from 20% to 80% in Ireland, and from 40% to 80% in the UK, Australia, and New Zealand.

Ireland emerged as the country with the highest rates of drinking, both before (90%) and during (82%) pregnancy, and of binge drinking, before (59%) and during (45%) pregnancy, based on estimates from the SCOPE study. But the exact prevalence could be far lower than that as estimates of drinking during pregnancy from the PRAMS and GUI studies were substantially lower (20-46%), with only 3% of women reporting binge drinking in PRAMS, caution the researchers.

The amount of alcohol drunk varied across the three studies. Between 15% and 70% of the women said that they had drunk 1-2 units a week during the first three months (trimester) of their pregnancy. But the number of reported units dropped substantially in all countries between the first and second trimester, as did binge drinking.

The findings indicated that the prevalence of drinking while pregnant was generally evident across all social strata, but several factors were associated with a heightened or lowered risk of alcohol consumption.

Compared with white women, those of other ethnicities were less likely to drink alcohol while pregnant, while younger women (30-39) were also less likely to do so than older women.

A higher level of education, having other children, and being overweight/obese were also associated with a lower risk of drinking while pregnant.

But the strongest and most consistent predictor of a heightened risk of drinking alcohol during pregnancy across all three studies was smoking. Smokers were 17-50% more likely to drink while pregnant.

The researchers point out that most clinical and government guidelines advise women to stop drinking during pregnancy.

But they write: “Alcohol use during pregnancy is highly prevalent, and evidence from this cross-cohort and cross-country comparison shows that gestational alcohol exposure may occur in over 75% of pregnancies in the UK and Ireland.”

However, most of these women consumed alcohol at very low levels and the number of pregnant women who drank heavily in the three studies was small, they say.Nevertheless, given that the risks of light drinking are not fully known, the most sensible option is not to drink alcohol during pregnancy, they add.

“Since most women who consume alcohol do so at lower levels where the offspring growth and development effects are less well understood [than at higher levels], the widespread consumption of even low levels of alcohol during pregnancy is a significant public health concern,” they conclude.