The evidence for the potentially harmful effects of light or occasional drinking in pregnancy is 'surprisingly limited,' but women are still better off avoiding all alcohol while pregnant, just in case, concludes a pooled analysis of the available data, published in the online journal BMJ Open.
The UK Chief Medical Officer recently commissioned a review of current alcohol guidelines for the general public and also specifically for mums-to-be, which resulted in a proposal to recommend that women should not drink any alcohol at all while trying to conceive or while pregnant, on the grounds that it is 'better to be safe than sorry.'
Women often ask about 'safe' levels of drinking during pregnancy, but there are no clinical trial data on this issue. In response, the researchers systematically reviewed all the data from a wide range of high quality observational studies on the impact of light drinking (two units up to twice a week, or four units a week, equivalent to a total of around 32 g) 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, impaired intellect, and behavioural difficulties typical of fetal alcohol syndrome—a consequence of heavy drinking in pregnancy.
From among nearly 5000 articles, they selected 26 relevant studies with data suitable to be pooled.
The analysis showed that drinking up to four units a week while pregnant, on average, was associated with an 8 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.
For most of the outcomes the researchers analysed, there were only a few studies that compared light to non-drinkers.
The issue remains of great public health importance, say the researchers, because up to 80 per cent of mums-to-be in the UK, Ireland, New Zealand and Australia drink some alcohol during their pregnancy.
But the evidence on how much, if any, is safe to drink, or at what stages of pregnancy, is notable by its absence, they add.
The lack of high quality data illustrates the difficulties of designing research that can truly evaluate the causal impact of light drinking while minimising the risks of bias and confounding, say the researchers.
And it also illustrates the failure of researchers so far to focus on 'light' drinking versus no drinking, rather than just on moderate and heavy drinking - a question many expectant mums care very much about ('But one glass is OK, isn't it?'), they add.
"Despite the distinction between light drinking and abstinence being the point of most tension and confusion for health professionals and pregnant women, and contributing to inconsistent guidance and advice now and in the past, our extensive review shows that this specific question is not being researched thoroughly enough, if at all," they write.
In the absence of any strong evidence, advice to women to steer clear of alcohol while pregnant should be made on the basis that it is a precautionary measure, they say.
"Women who have had a drink while pregnant should be reassured that they are unlikely to have caused their baby considerable harm, but if worried, they should discuss this with their GP or midwife," say the researchers.
They conclude: "Evidence of the effects of drinking up to 32 g/week in pregnancy is sparse. As there was some evidence that even light prenatal alcohol consumption is associated with being SGA [underweight] and preterm delivery, guidance could advise abstention as a precautionary principle."
Explore further: Drinking alcohol while pregnant is common in UK, Ireland, and Australasia
Low alcohol consumption and pregnancy and childhood outcomes: time to change guidelines indicating apparently 'safe' levels of alcohol during pregnancy? A systematic review and meta-analysis, BMJ Open (2017). DOI: 10.1136/bmjopen-2016-015410