Is it all right for women to drink small amounts of alcohol in pregnancy? No

Abstract

The National Institute for Health and Clinical Excellence draft guidance on drinking during pregnancy has added to confusion about safety. Pat O'Brien believes that telling women to abstain is overly paternalistic on current evidence, but Vivienne Nathanson and colleagues argue that this is the safest message

The latest government advice in England says that pregnant women and women trying to conceive should avoid alcohol. A new BMA report, Fetal alcohol spectrum disorders, agrees, recommending that abstinence is the only safe policy for women who are pregnant or planning a pregnancy.¹ This view is shared by the Royal College of Obstetricians and Gynaecologists² and the World Health Organization.³ In the United States, the surgeon general recommends that women who are pregnant or who may become pregnant should abstain from consuming alcohol.⁴

Damage from alcohol

Alcohol can adversely affect the reproductive process in several ways, including infertility, miscarriage, preterm deliveries, stillbirth, and low birthweight babies.^{5 6 7} Alcohol is teratogenic and readily crosses the placenta. Because the fetus does not have a developed blood filtration system, it is unprotected from alcohol in the bloodstream. The damage caused by alcohol depends on the level of maternal alcohol consumption, the pattern of alcohol exposure, and the stage of pregnancy.⁸ Other risk factors include the genetic makeup of the mother and the fetus, the nutritional status of the mother, hormonal interactions, use of other drugs (including tobacco), general health of the mother, stress, maternal age, and low socioeconomic status.^{9 10}

Human and animal studies provide robust and consistent evidence that heavy maternal alcohol use is associated with fetal alcohol syndrome.^{11 12} Research has identified vulnerable periods of neonatal development that can be adversely affected by exposure to heavy doses of alcohol.^{8 9 11 12} Evidence from animal experiments suggests these critical periods of exposure occur during the first and third trimesters in humans.¹³

There is, however, considerable debate about the adverse effects of low to moderate maternal alcohol consumption. This may be explained by the variability in the definitions of consumption levels, differences in the way drinking behaviour is characterised, and methodological problems in the design and analysis of relevant studies and in determining the relative effect of confounding factors including genetic predisposition. There is currently no consensus on the level of risk or whether there is a clear threshold below which alcohol is non-teratogenic. Because there is no conclusive evidence that one to two units of alcohol a week is harmful to the developing fetus,^{5 10 11} the guidance has until recently recommended an upper limit of this amount.

Evidence is continuing to emerge on the possible risks of prenatal alcohol exposure at low to moderate levels. ^{10 14 15 16} Animal experiments suggest that damage to the central nervous system may occur at low levels of alcohol exposure.^{9 17 18} A prospective study of 501 mother-child dyads found that the child's behaviour at age 6-7 years was adversely related to low to moderate levels of prenatal alcohol exposure.¹⁴ A dose-response relation between the alcohol consumed and behaviour was also found.¹⁴ Studies examining the effects of alcohol on the fetus have shown that acute exposure to one to two units of alcohol causes a rapid decrease in fetal breathing.^{19 20} Studies examining the effects of chronic consumption indicate that low to moderate levels of exposure (two to five units a week) delay the development of the fetus's nervous system and may have a permanent effect.¹⁵ It is not clear what effect changes in behaviour have on fetal development. A recent large prospective study has found that occasional low to moderate drinking during the first trimester may have a negative and persistent effect on children's mental health.¹⁶

Confusion about consumption

Countries such as the US, Canada, France, New Zealand, and Australia have adopted the abstinence message. Current guidelines on sensible drinking in the UK can be misinterpreted as people may not clearly understand what units or “standard drinks” are.²¹ This is compounded by variation in the alcoholic concentration of different types of drink, variation in serving size (such as different sizes of wine glass), and the difference between the standard measures used in bars and restaurants as well as measures poured in the home.²¹ Many women will not know they are pregnant during the early part of the first trimester, during which time they may continue to drink in their pre-pregnancy fashion with no awareness of the risk to their unborn child. Given the current uncertainty regarding the level of risk to the developing fetus, the lack of clear guidelines, and the confusion about consumption levels, the only sensible message for women who are pregnant or planning a pregnancy must be complete abstinence from alcohol.