“There is little evidence having the occasional drink while pregnant harms a baby,” reports the Mail Online.

This follows a review of international research looking at whether low-to-moderate alcohol consumption – no more than 1 to 2 units, once or twice a week – was linked with adverse pregnancy outcomes. To put this in context, a pint of low-strength lager contains about 2 units of alcohol, a small glass (125ml) of 12% wine contains 1.5 units and a single shot of spirits contains 1 unit.

The review found that low-to-moderate alcohol consumption may slightly increase the risk of having a baby small for gestational age.

The Chief Medical Officers for the UK currently recommend that pregnant women, or those planning to become pregnant, avoid drinking any alcohol at all to keep risks to a minimum. Long-term health risks for the baby are greater the more alcohol you drink.

And as the researchers conclude, no evidence of harm is not the same as evidence of no harm: there could still be risks that haven’t been identified.

However, the research does provide reassurance that women who have drunk small amounts of alcohol during pregnancy, or before they knew they were pregnant, are unlikely to have harmed their baby.

Where did the story come from?

The study was carried out by researchers from the University of Bristol in the UK. It was funded by the UK Medical Research Council, the University of Bristol, the National Institute for Health Research (NIHR) Collaboration for Leadership in Applied Health Research and Care West (CLAHRC West) at University Hospitals Bristol NHS Foundation Trust, and the Higher Education Funding Council for England.

The study was published in the peer-reviewed medical journal BMJ Open on an open-access basis, meaning it’s freely available to read online.

Media reporting of the study was generally accurate and responsible, making it clear that it’s probably still best to avoid alcohol during pregnancy.

Saying as The Guardian does, however, that “women worried by guidance advising abstinence should be told there is little evidence that the odd glass of wine causes harm to the baby” is a little misleading, as it could be interpreted to mean that there’s definitely no harm in drinking. The reality is that there isn’t enough research being done in the area, so we cannot firmly say whether or not it may be harmful.

What kind of research was this?

This was a systematic review and meta-analysis study looking at the effect of low-to-moderate alcohol intake in pregnancy (fewer than 2 UK units, no more than twice a week) on pregnancy and longer-term infant outcomes.

While this is a good way of summarising all evidence on a particular issue, systematic reviews will include any limitations of the studies reviewed. It wouldn’t be ethical to randomise pregnant women to consume alcohol or not, so most studies are observational – such research cannot prove that a certain level of alcohol intake has directly caused any adverse outcomes.

It’s also difficult to be sure of exact alcohol intake, and a range of other health and lifestyle factors may contribute to the outcome too.

What did the research involve?

Researchers looked at 24 cohort and two quasi-experimental studies (which don’t involve randomisation) from a range of high-income countries, including the UK.

Low-to-moderate alcohol intake was defined as 32g a week (1 or 2 units, once or twice a week) as this was the “safe” threshold cut-off previously specified by UK guidelines. There has been a recent move among guideline development groups to recommend total abstinence during pregnancy.

Pregnancy outcomes assessed included:

• stillbirth (pregnancy loss after week 24)
• miscarriage (loss before week 24)
• pregnancy duration and premature delivery (less than 37 weeks)
• hypertensive disorders of pregnancy
• gestational diabetes
• baby born small for gestational age
• birth size (weight, length and head circumference)
• low birth weight (under 2.5kg)
• low amniotic fluid, placenta previa and placental abruption
• assisted delivery (using forceps, for example)
• Apgar score at birth and admission to neonatal unit (Apgar scores assess the health of a newborn on a scale from 1 to 10, based on factors such as heart rate and breathing patterns)
• congenital malformation

They also looked at features of foetal alcohol syndrome disorder (birth defects caused by heavy alcohol consumption in pregnancy), including childhood growth restriction, head size and circumference, developmental delays, behaviour problems, cognitive impairment and IQ, and facial malformations.

Researchers looked at whether study results had been adjusted for potential confounders, including socioeconomic status, smoking during pregnancy, maternal age and ethnicity.

What were the basic results?

Of the 24 studies, 17 were suitable to be pooled in a meta-analysis for four outcomes: birth weight, small for gestational age, premature birth and low birth weight (under 2.5kg).

There was modest evidence that low-to-moderate alcohol consumption gave an 8% increased risk of being small for gestational age compared with abstinence (odds ratio 1.08, 95% confidence interval 1.02 to 1.14).

This was from pooled results of seven studies, although 95% of the participants came from a single US study.

Low-to-moderate alcohol consumption had no significant effect on the likelihood of a low birth weight (six studies) or premature birth (nine studies). Seven studies also found no significant difference in the average infant birth weight between drinkers and non-drinkers.

For all other outcomes, there was not enough data to combine the results or reach firm conclusions.

How did the researchers interpret the results?

The researchers concluded there is “limited evidence for a causal role of light drinking in pregnancy, compared with abstaining, on most of the outcomes examined”.

They added: “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.”

Conclusion

The results of this review found that low-to-moderate alcohol consumption during pregnancy was linked with a slightly increased risk of having a baby small for gestational age.

However, there was no evidence for any other links, including any difference in the average birth weight of babies born to drinkers and non-drinkers.

There are some important limitations of the research to note:

• The evidence still doesn’t prove that drinking directly increases the risk of a baby born small for gestational age. Studies were observational and varied widely in accounting for the extensive number of confounding factors that could be having an influence, such as maternal lifestyle and diet. And even when factors such as socioeconomic status were adjusted for, they may still have had some residual confounding effects.
• Because studies were observational, it’s hard to be certain of exact alcohol intake. It may have varied week to week, and many women may not have been able to accurately judge the number of units of alcohol they had.
• As the authors acknowledge, there was little evidence available for many pregnancy and birth outcomes, and the studies weren’t suitable for pooling. As such, we can’t be sure that being small for gestational age – if this is a true risk – is the only one associated with drinking during pregnancy.

The research generally supports the opinion that it’s difficult to say what a “safe” amount of alcohol is for women who are pregnant or trying for a baby.

The current advice from UK Chief Medical Officers is that, if pregnant or planning a pregnancy, the safest approach is not to drink alcohol at all. They say drinking in pregnancy can lead to long-term harm to the baby, with the risk becoming greater the more you drink.

They also advise that women who find out they’re pregnant after already having drunk in early pregnancy should avoid further drinking but shouldn’t worry unnecessarily, as the risks of their baby being affected are likely to be low.

Links To The Headlines

Little evidence that light drinking in pregnancy is harmful, say experts. The Guardian, September 11 2017

‘Weak evidence’ light alcohol use in pregnancy harms. BBC News, September 12 2017

Light drinking in pregnancy ‘unlikely’ to harm babies, but experts caution lack of evidence. The Daily Telegraph, September 11 2017

Light drinking during pregnancy ‘may be OK’ as researchers find limited evidence of harm. The Sun, September 12 2017

‘No proof’ that the odd glass of wine harms your baby, despite Government advice to abstain while pregnant. Mail Online, September 12 2017

Links To Science

Mamluk L, Edwards HB, Savovic J, et al. 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-analyses. BMJ Open. Published online September 11 2017