Many women who consume low to moderate levels of alcohol continue to do so in the first month or so of pregnancy - often because they do not realize they are pregnant.
Senior author Michael B. Bracken, a professor of epidemiology at the Yale School of Public Health in New Haven, CT, and colleagues report their findings in the Annals of Epidemiology.
Health experts warn that drinking alcohol during pregnancy carries a potential risk of harming the baby. Heavy alcohol consumption during pregnancy is known to be linked to fetal alcohol syndrome and other birth defects.
While some professionals may unofficially say it is fine for pregnant women to indulge in the occasional drink, in many countries - including the US - the official advice is women should avoid alcohol altogether when trying for a baby and while pregnant, because there are no known quantities considered safe.
However, many women who consume low to moderate levels of alcohol continue to do so in the first month or so of pregnancy - often because they do not realize they are pregnant - and the researchers wanted to find out more about the risks for this group.
For their study, the team analyzed links between alcohol consumption and birth outcomes data on nearly 4,500 women and their babies. Their analysis included information on low birth weight, preterm delivery and growth restriction while in the womb.
They also looked at other birth outcomes such as newborns' birth length and head circumference and whether they were admitted to intensive care units, were jaundiced or had any major birth defects.
Nearly a third of women said they drank during the first month of pregnancy
The analysis showed that around 30% of the women reported drinking alcohol during the first month of their pregnancy. The most commonly reported alcoholic drink was wine.
Overall, the levels of alcohol consumed among women who said they drank during their pregnancy were low. For the first month of pregnancy, the median level of alcohol consumption was about one drink per week.
The researchers found there was a reduced chance of babies being of low weight, of low length and having a small head circumference when they born to women who drank low to moderate levels of alcohol in early pregnancy. All these features are hallmarks of fetal alcohol syndrome.
The study also found that drinking in the third trimester was linked to a lower risk of low birth weight and preterm delivery.
More research needed to confirm whether low alcohol intake in pregnancy is harmless
Prof. Bracken says their results are consistent with those of other studies that show there is no evidence low to moderate levels of alcohol consumption in pregnancy cause harm, and more research should be done to investigate this apparent contradiction.
He notes that the findings also show that alcohol consumption is most prevalent during the first month of pregnancy - a period of early fetal development - when many women do not realize they are pregnant.
"Whether these associations of reduced risk are due to healthier lifestyles of women consuming low to moderate alcohol and especially wine cannot be fully ruled out," explains Prof. Bracken.
He adds that although the study stopped short of looking at links between heavy or chronic alcohol consumption in pregnancy and alcohol-related birth outcomes like fetal alcohol syndrome, it nevertheless makes an important contribution in that it examines more typical levels of alcohol consumption during pregnancy.
The National Institutes of Health funded the study.
Medical News Today recently reported on a new study that, for the first time, identifies preterm birth as a leading cause of death in young children. Of the 6.4 million deaths among children under the age 5 in 2013, worldwide, nearly 1.1 million were due to complications tied to preterm birth.