The occasional glass of wine or champagne during pregnancy cannot be considered safe due to a lack of evidence and alcohol must be avoided, researchers warn.
A systematic review of all available research on the impact of low levels of drinking in pregnancy, published in BMJ Open, has found there is a lack of evidence to support a 'safe' level of maternal consumption of alcohol.
Because the evidence is sparse abstinence from alcohol during pregnancy is recommended, the British authors say.
Australian expert Elizabeth Elliot AM, Professor of Paediatrics and Child Health at the University of Sydney and co-director of the NHMRC Centre of Excellence in Fetal Alcohol Spectrum Disorder, agrees.
"Even though there may be not obvious risk from low levels of alcohol our advice as health professionals must be the safest option is to avoid alcohol," Prof Elliot said.
"We know that alcohol can cause harm both at a cellular level and a clinical level therefore the precautionary approach is safest and one of the reasons is that often people who are given the go-ahead to drink will drink more than they are advised to drink."
British researchers looked at a total of 5,000 observational studies and only 26 had data on the impact of light drinking - that is two units up to twice a week or four units a week (less than 32 grams) - during pregnancy compared with no alcohol at all.
They looked particularly at complications of pregnancy and birth characteristics, such as miscarriage, premature birth, and undersized babies, and longer term issues, such as the developmental delays.
The analysis showed that drinking up to four units a week while pregnant, on average, was associated with an eight per cent higher risk of having a small baby, compared with drinking no alcohol at all .
There was also some evidence of a heightened risk of premature birth, but this was less clear.
Professor Jane Halliday from the Murdoch Children's Research Institute says the study has its limitations but the message that low level drinking in pregnancy is OK is ill-advised.
"We recently published a paper that shows that even low amounts of alcohol consumed in trimester one contribute to facial shape in one-year-olds," Prof Halliday said.
The impact on facial shape, published in journal JAMA Pediatrics, were not visible to the naked eye and only seen by using sophisticated 3D facial shape analysis
Despite this, the slight changes are still significant because children who have been exposed to high levels of alcohol in pregnancy have a "very classic face", Prof Halliday said.
"What we were seeing was very subtle manifestations of those differences in the middle of the face."