Almost half of all Australian mothers-to-be drink alcohol throughout their pregnancy, and some even admit to binging in the final months before giving birth.
The women who were prepared to drink on were also more likely to smoke during their pregnancy, according to a study of 4,700 mothers in Western Australia.
Doing so placed these women at the most risk of having their baby prematurely, according to the research by WA's Telethon Institute for Child Health Research.
"Our research shows pregnant women who drink more than one to two standard drinks per occasion - and more than six standard drinks per week - increase their risk of having a premature baby," says institute researcher Colleen O'Leary.
This was the case even if the women stopped drinking before their second trimester, Ms O'Leary said.
While there was no difference for women who abstained or drank low levels of alcohol, it said abstinence was still the safest option.
"The risk of pre term birth is highest for pregnant women who drink heavily or at binge levels, meaning drinking more than seven standard drinks per week, or more than five drinks on any one occasion."
The study focused on a random selection of non-indigenous women who gave birth between 1995 and 1997, and they were quizzed on their pregnancy and pre-pregnancy drinking habits.
Fewer than 20 per cent of women abstained during the pre-pregnancy period, but this increased to 57 per cent in the first two trimesters before settling to 54 per cent in the third trimester.
"Low" or "moderate" drinking came in at 44 per cent during the third trimester, while more than two per cent of women admitted to "binge" or "heavy" drinking in the final months before birth.
The study found a low birth weight was more likely to be caused by a mother's smoking rather than drinking.
And, while there was no difference for women who abstained or drank low levels of alcohol, it said abstinence was still the safest option.
""Women should be advised that during pregnancy, drinking alcohol above low levels increases the risk to the baby and that the safest choice is not to drink alcohol," Ms O'Leary said.
The study was also conducted jointly by the National Perinatal Epidemiology Unit at the University of Oxford.
The results are to be published in BJOG: An International Journal of Obstetrics and Gynaecology.
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