A new study suggests light alcohol consumption during the first trimester is linked to a higher risk of bearing a preterm or unexpectedly small baby, even if expectant mothers keep within the government-recommended limit of two units a week.
In addition, say researchers at the University of Leeds in the UK, who report their findings in the BMJ's Journal of Epidemiology and Community Health, drinking alcohol in the weeks leading up to conception is also linked to smaller fetal growth.
The new finding adds to a debate, fueled by mixed evidence, that has been going on for some time about light alcohol consumption in pregnancy.
The authors note that when pregnant women drink alcohol, it crosses the placenta and results in nearly equal cocentrations in both the fetus and the mother.
However, while the effects of heavy drinking in pregnancy are well established, the effect of light drinking is not, and different studies have come to different conclusions about it, as have different health authorities.
Varied evidence and advice about light alcohol consumption in pregnancy
For example, in 2010, the same journal published a study led by University College London, which tracked children up to the age of 5 and found no evidence that light drinking in pregnancy harms children's development.
But in 2012, researchers in the US who were concerned about fetal alcohol syndrome - a range of growth, mental and physical abnormalities that can occur in babies whose mothers drink alcohol during pregnancy - said there was no safe limit for alcohol consumption in pregnancy. They described how they came to this conclusion in a paper published in the journal Alcoholism: Clinical and Experimental Research.
And then, a year later, another UK study published in BMJ Open looked at the effects drinking during pregnancy might have on fetal development and concluded that moderate alcohol consumption in pregnancy was safe.
Meanwhile, American women are advised by the US Centers for Disease Control and Prevention (CDC) that there is no safe level of alcohol use during pregnancy, and that women who are pregnant or plan on becoming pregnant should refrain from drinking alcohol.
This is also the view of the Society of Obstetricians and Gynecologists of Canada, which recommend universal screening for alcohol consumption for all pregnant women and women of childbearing age.
The UK's Department of Health advice is less strict. They recommend women who are pregnant or trying to conceive should not drink alcohol at all, but if they choose to drink, then they should limit their consumption to one or two units, once or twice a week. That is no more than a pint of beer, or a medium glass of wine, twice a week.
Drinking over two units a week ups chances of bearing preterm or small babies
For this latest study, the Leeds researchers examined data on 1,264 pregnant women in Leeds aged between 18 and 25 who were assessed to be at low risk for birth complications.
All the women were enrolled in the Caffeine and Reproductive Health (CARE) Study, and had filled in food frequency questionnaires that asked them to specify how often they drank alcohol and of what type, 4 weeks before conception and during each trimester throughout their pregnancy.
When their babies were born, 13% were underweight and 4.4% were smaller than would be expected, while a similar percentage (4.3%) were preterm.
Even after taking into account other possible influencers, when they analyzed the alcohol data against the birth outcomes, the researchers found women who drank more than two units of alcohol per week were twice as likely to give birth to a premature or unexpectedly small baby than women who did not drink alcohol at all.
But the analysis showed that even women who did not drink more than the maximum recommended alcohol intake during this period were still at increased risk of giving birth prematurely.
Plus, drinking during the period leading up to conception was also associated with a risk of restricted fetal growth, suggesting this may also be a critical time, note the authors.
During pregnancy, average intake was 11 units a week in first trimester
The data showed that on average, the women drank the most alcohol before conception and in the first trimester. During pregnancy, the average intake was 11 units a week in the first, four in the second, and just under two in the last trimester.
More than half (53%) of the moms-to-be drank more than two units a week in the first trimester, and nearly 40% said they drank more than 10 units a week in the weeks leading up to conception.
The study also found expectant mothers who were the most likely to drink more than the recommended limit of two units a week tended to be older, white, better educated and living in more affluent areas.
Senior author Janet Cade, a professor in Leeds' School of Science and Food Nutrition, says:
"Since pregnancy is such a special time for women it would be sensible for anyone who is thinking about getting pregnant to avoid alcohol during that time."