Drinking during pregnancy can cause fetal alcohol spectrum disorders (FASD) – lifelong physical, intellectual and behavioural disabilities – with a report from the US Centers for Disease Control and Prevention highlighting that the damage can be done before women even realise they are pregnant.

The report estimated that 3.3 million American women risked exposing their developing baby to alcohol because they were drinking, sexually active and not using birth control.

Australian guidelines recommend that women who are pregnant or planning a pregnancy do not drink at all. But 40 per cent of Australians report drinking at least some alcohol while pregnant, and new research has found that 60 per cent of pregnant women drank before realising they were expecting.

Professor of Paediatrics and Child Health at the University of Sydney, Elizabeth Elliott, said drinking was "one of the worst things you can do during pregnancy".

"We know alcohol passes across the placenta and is a direct toxin to the baby's brain and other organs," she said. "We know that about half of pregnancies in Australia are not planned. So my message to people is: if you're drinking, use contraception. If you're trying to get pregnant, don't drink. The risk to the embryo and fetus is great in that first six to eight weeks of gestation when the brain and the other organs are developing."

Professor Elliott said GPs and other primary health workers should be educating their patients on the risks of alcohol consumption in pregnancy before people started trying for a baby, and on the importance of contraception if they did drink.

"I think that some health professionals take the attitude that they can't be paternalistic and that they don't want to upset or frighten people," she said. "But if you speak to women they want the best information and the very best outcome for their baby."

Professor Elliott said the more a pregnant woman drank, the greater the risk of harm. But risk was difficult to predict in individuals because factors like age, health, liver function and metabolism influenced maternal - and hence fetal - blood alcohol levels. No safe level of alcohol intake during pregnancy has been or ever will be established, she said.

Even when a pregnancy is planned, it is four to six weeks before most women find out they are expecting. That time is "a high risk for alcohol exposure", said Dr Clare McCormack, from the National Drug and Alcohol Research Centre (NDARC).

According to a study of 1600 pregnant women in Sydney and Perth by NDARC and the National Drug Research Institute, almost 60 per cent drank alcohol between conceiving and learning of the pregnancy.

"We found that drinking at binge or heavy levels in [that]  period ... was more common than drinking at low levels," Dr McCormack said. "Binge drinking is common in our society so it's not surprising that it's common in that time period too."

Drinkers tended to be older, better educated, from higher socio-economic backgrounds and less likely to have planned their pregnancies. They were also more likely to be in their first pregnancy.

Nineteen per cent did not give up alcohol after discovering they were pregnant, but Dr McCormack said most were "only drinking at low levels ... maybe on one or two occasions".

Paediatrician Dr Doug Shelton, Director of Community Child Health at Gold Coast Health and a specialist in child development and behavioural problems, said the community needed to be better educated about the dangers of alcohol, beginning in high school.

"In our culture binge drinking is very common, and we know that binge drinking when you're pregnant is the single most problematic type of drinking," he said. "Someone who binge drinks while pregnant is more likely to have a baby with FASD. [Drinking] is unsafe throughout the entire pregnancy but it is especially unsafe in that first three months."

Dr Shelton said women's partners had an important role to play, with movements such as Pregnant Pause encouraging men to share the responsibility for avoiding alcohol. "Evidence shows that in instances where a mother did drink during pregnancy, often that drinking was initiated by her partner," he said.

FASD is most prevalent in white middle class families, Dr Shelton said. At the most serious end of the spectrum is fetal alcohol syndrome, affecting a child's growth, neurological development and producing characteristic facial abnormalities. About 85 per cent of babies with neurological damage from FASD look normal, Dr Shelton said, so intellectual and behavioural problems may go undetected for several years. Only 20 per cent of those with FASD are able to live independently as adults.

The National Organisation for Fetal Alcohol Spectrum Disorders chief executive, Vicki Russell, said it was important to advise, rathe than stigmatise, women who drink while pregnant and to support them to reduce their alcohol intake.

Women called the organisation, worried because they drank before realising they were pregnant, and it was a "devastating time" for them, Ms Russell said. "Unfortunately we can't give them an answer, we can't say, 'You'll be fine'. We don't know which unborn child is vulnerable, which pregnancy is vulnerable."

But FASD could be totally avoided, Ms Russell said. "Why take the risk when it's something you can prevent by not drinking? No alcohol, no FASD."