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Evidence for potential harms of light drinking in pregnancy ‘surprisingly’ limited
But review confirms alcohol best avoided ‘just in case’, say researchers
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.”
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Elizabeth Elliott is a Professor of Paediatrics and Child Health at the University of Sydney and Co-director of the NHMRC Centre for Research Excellence in Fetal Alcohol Spectrum Disorder
This is a well-conducted systematic review with meta-analysis designed to eliminate bias, which focuses on the effects of light drinking (≤32g/week) during pregnancy.
The conclusions are not new: there is a paucity of good quality data on effects on offspring of low-level drinking during pregnancy. Current Australian research in two pregnancy/birth cohorts is addressing this question (Muggli et al 2016; McCormack et al 2017).
Although we found no association between low level exposure and neurodevelopmental outcomes (Halliday et al 2017) we found even low levels of prenatal alcohol exposure are associated with subtle facial abnormalities on 3-D imaging, reflecting an effect on the brain (Muggli et al 2017).
The BMJ authors found significantly higher risk of small for gestational age (SGA) in babies exposed to three or fewer Australian standard drinks per week compared to unexposed babies. This is important because SGA has later health consequences including increased risk of cardiovascular and metabolic disease.
I agree with the precautionary approach recommended because:
- We will never conduct experiments on pregnant women to determine a ‘safe’ level of alcohol intake. However, we know alcohol is a teratogen that harms the brain with potential severe consequences.
- A lack of evidence of harm does not mean there is a lack of harm. Epidemiology is a ‘blunt’ tool and animal models suggest that harms from alcohol may be invisible (e.g. changes to nerve migration, apoptosis of brain cells, production of neurotransmitters). It is not logical to recommend any intake of a potential teratogen.
- Harm to the fetus depends on factors apart from alcohol dose e.g. timing, duration of exposure. The blood alcohol level in the mother (and hence fetus) depends on factors including maternal genetics for metabolising alcohol and is influenced by maternal age, body composition, health and liver function.
- This precautionary advice is consistent with NHMRC Australian guidelines."
- Muggli E, O'Leary C, Donath S, Orsini F, Forster D, Anderson PJ, et al. "Did you ever drink more?" A detailed description of pregnant women's drinking patterns. BMC public health. 2016;16:683.
- McCormack C, Hutchinson D, Burns L, Wilson J, Elliott E, Allsop S, et al. Prenatal Alcohol Consumption Between Conception and Recognition of Pregnancy. Alcoholism, clinical and experimental research. 2017;41(2):369-78.
- Halliday JL, Muggli E, Lewis S, Elliott EJ, Amor DJ, O'Leary C, Donath S, Forster D, Nagle C, Craig JM, Anderson PJ. Alcohol consumption in a general antenatal population and child neurodevelopment at 2 years. J Epidemiol Community Health. 2017 Aug 24. pii: jech-2017-209165. doi: 10.1136/jech-2017-209165.
- Muggli E, Matthews H, Penington A, Claes P, O’Leary C, Forster D, Donaht S, Anderson PJ, Lewis S, Nagle C, Craig JM, White SM, Elliott Ej, Halliday J. Association between prenatal alcohol exposure and craniofacial shape of children at 12 months of age. JAMA Pediatr. 2017 Jun 5. Doi: 10.1001/jamapediatrics.201720778. PMID: 28586842
Professor Jane Halliday is a group leader in Public Health Genetics, from the Murdoch Children's Research Institute
We agree that there is no good evidence for an adverse association between low level drinking in pregnancy and the perinatal outcomes that these authors have examined, but what about other outcomes not mentioned in this paper?
For instance, we have recently published a paper that shows that even low amounts of alcohol consumed in trimester one contribute to facial shape in one year olds. These changes are not visible to the naked eye and are only seen by using sophisticated 3D facial shape analysis.
At the moment, we don’t know what these changes mean clinically and plan to follow up the children’s development as they reach school age. It is well recognised that child neurodevelopment is a key outcome that cannot be dismissed as being adversely affected by any level of alcohol exposure in pregnancy.
A message that low level drinking in pregnancy is OK is ill-advised and guidelines should not be changed.
Dr Delyse Hutchinson is a Theme Leader in the Centre for Social and Early Emotional Development at Deakin University & the National Drug & Alcohol Research Centre at the University of New South Wales (UNSW); Professor Richard P. Mattick is the Professor of Drug and Alcohol Studies in the National Drug & Alcohol Research Centre at the University of New South Wales (UNSW)
This review highlights the need for science to address a key gap in the health literature - that of light drinking in pregnancy and potential effects on child outcomes. Without question, we know that heavy prenatal alcohol exposure is harmful to fetal development. The issue of lower level exposure is considerably more complex and one which warrants focused research attention.
Our prevalence data show that many women in Australia consume alcohol at some stage during pregnancy. Much of this consumption occurs at levels considered to be low, and commonly, prior to pregnancy awareness, in the early stages of a pregnancy (1,2). Given current inconsistency in the literature (and the marked paucity of research) on the potential harms of alcohol use at low levels, the safest current advice to women is to avoid alcohol in pregnancy.
Two NHMRC longitudinal cohort studies in Australia have been studying this issue, with particular focus on low-level exposure (The Triple B Pregnancy Study and Asking Questions about Alcohol in Pregnancy). The results of these studies will help inform the issue and add to the empirical basis for health advice to women on alcohol use in pregnancy."
- Hutchinson D, Moore EA, Breen C, et al. Alcohol use in pregnancy: Prevalence and predictors in the Longitudinal Study of Australian Children. Drug and Alcohol Review 2013 doi: 10.1111/dar.12027
- McCormack C, Hutchinson D, Burns L, et al. Prenatal alcohol consumption between conception and recognition of pregnancy. Alcoholism: Clinical and Experimental Research 2017;41(2):369-78. doi: 10.1111/acer.13305.
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