Government recommendations that women should stop drinking altogether in pregnancy was based on “generally weak” and “limited” evidence, claim Bristol University scientists.
However, they advise expectant mothers to avoid alcohol “just in case” as there was a small risk of having an underweight or premature baby linked to drinking four units a week, or 32g of alcohol, or about two glasses of wine.
A standard 175ml glass of wine with an alcohol by volume, or ABV, of 12 per cent, contains 2.1 units.
Previous studies showed heavy drinking during pregnancy could cause birth defects, affected the baby’s intelligence and could lead to behavioural, mental and fine motor problems.
As a result women were told to avoid heavy or binge drinking but advice on safe levels remained a grey area.
Until recently UK guidelines advised women they could drink up to one or two units, once or twice a week. However, new guidelines from the UK Chief Medical Officer recommended women do not drink any alcohol while trying to conceive or when pregnant to be “better safe than sorry”.
But this has left women confused about a safe limit and whether just one glass was harmful.
Up to 80 per cent of mums-to-be in the UK, Ireland, New Zealand and Australia are believed to drink some alcohol during their pregnancy.
The Bristol University study, published in the BMJ Open journal, looked at links between low-to-moderate levels of drinking during pregnancy and long-term health effects on children.
It found 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 to drinking no alcohol, as well as a 10 per cent increased risk of premature birth.
Researcher Dr Loubaba Mamluk said: “We found limited evidence for a causal role of light drinking in pregnancy compared with abstaining.
“Evidence of the effects of drinking up to 32g a week in pregnancy is sparse. Women who have had a drink while pregnant should be reassured they are unlikely to have caused their baby considerable harm.”
But she added: “As there was some evidence that even light prenatal alcohol consumption is associated with being underweight and pre-term delivery, guidance could advise abstention as a precautionary principle.”