No adverse effects from light drinking in pregnancy but abstention still advised

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A research review suggests that consuming even small amounts of alcohol in pregnancy may be linked with higher chances of having a small baby and delivering prematurely, but that there were actually very few studies investigating light drinking during pregnancy.

Research from the University of Bristol has reviewed all the high quality scientific studies they could find on the effects of drinking small amounts of alcohol during pregnancy. Small amounts were defined as 1 to 2 UK units, once or twice a week, the recommended maximum level given in the UK Department of Health’s previous guidance. Two units of alcohol is equivalent to one pint of strong beer or a medium size glass (175ml) of light white wine.

They looked at the effects on a wide range of health outcomes such as miscarriage, still birth, size at birth, and long term developmental delays, behavioural and cognitive deficits.

The review found only seven studies with data on whether consuming small amounts of alcohol in pregnancy affected the size of babies at birth. Based on these studies, Dr Loubaba Mamluk and colleagues found evidence that women who reported drinking even this amount of alcohol were 8% more likely to deliver a small baby with estimates ranging between a 2% and a 14% increase. The evidence that light drinking affected delivering prematurely was weaker and there was insufficient evidence on other health outcomes.

This evidence supports the new guidelines released by the Department of Health in January 2016, which advise women not to drink any alcohol when pregnant or trying to conceive: “the safest approach is not to drink alcohol at all”. This was based on a “better safe than sorry” principle, in the absence of strong evidence for or against actual harmful effects of alcohol to the unborn baby.

Drinking large amounts of alcohol is clearly linked to foetal alcohol syndrome (FAS) and increases the risk of miscarriage, premature birth and the baby having a low birth weight.

The Bristol authors emphasise that the evidence they have analysed refers to light drinking, that the effects they have found are small, and that overall there is still some uncertainty, for example on how light drinking in pregnancy affects other aspects of the baby’s health.

Another reason to be cautious is that the association might not reflect a direct causal link. Women can use this new information to inform their choices, and if they want to avoid any risk from alcohol drinking in pregnancy or when trying to conceive they should avoid all alcohol as a precaution.

According to Dr Luisa Zuccolo, who co-led the study: “Formulating advice on the basis of the current evidence is challenging because we are still building the full picture of what happens to the unborn baby when small amounts of alcohol reach the bloodstream or brain. We wanted to give women the most up-to-date and reliable evidence in order to empower them to make an informed decision about drinking during pregnancy and balancing any possible risk with other factors in their lives.”

This research was funded by the National Institute for Health Research (NIHR) through the Collaboration for Leadership in Applied Health Research and Care West (CLAHRC West) and UK Medical Research Council through the MRC Integrative Epidemiology Unit at the University of Bristol.

Abstract
Objectives: To determine the effects of low-to-moderate levels of maternal alcohol consumption in pregnancy on pregnancy and longer-term offspring outcomes.
Search strategy: Medline, Embase, Web of Science and Psychinfo from inception to 11 July 2016.
Selection criteria: Prospective observational studies, negative control and quasiexperimental studies of pregnant women estimating effects of light drinking in pregnancy (≤32 g/week) versus abstaining. Pregnancy outcomes such as birth weight and features of fetal alcohol syndrome were examined.
Data collection and analysis: One reviewer extracted data and another checked extracted data. Random effects meta-analyses were performed where applicable, and a narrative summary of findings was carried out otherwise.
Main results: 24 cohort and two quasiexperimental studies were included. With the exception of birth size and gestational age, there was insufficient data to meta-analyse or make robust conclusions. Odds of small for gestational age (SGA) and preterm birth were higher for babies whose mothers consumed up to 32 g/week versus none, but estimates for preterm birth were also compatible with no association: summary OR 1.08, 95% CI (1.02 to 1.14), I2 0%, (seven studies, all estimates were adjusted) OR 1.10, 95% CI (0.95 to 1.28), I2 60%, (nine studies, includes one unadjusted estimates), respectively. The earliest time points of exposure were used in the analysis.
Conclusion: Evidence of the effects of drinking ≤32 g/week in pregnancy is sparse. As there was some evidence that even light prenatal alcohol consumption is associated with being SGA and preterm delivery, guidance could advise abstention as a precautionary principle but should explain the paucity of evidence.

Authors
Loubaba Mamluk, Hannah B Edwards, Jelena Savović, Verity Leach, Timothy Jones, Theresa HM Moore, Sharea Ijaz, Sarah J Lewis, Jenny L Donovan, Debbie Lawlor, George Davey Smith, Abigail Fraser, Luisa Zuccolo

University of Bristol material
BMJ Open abstract


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