It turns out that there's not much research on just how little a pregnant woman could drink without harming her unborn baby, according to a paper published in the journal BMJ Open on Monday.
In the United States, doctors have long warned that drinking any alcohol while pregnant can come with serious medical risks, such as the possibility of miscarriage, stillbirth, or physical and behavioral problems in the baby, known as fetal alcohol spectrum disorders.
Last year, a report from the Centers for Disease Control and Prevention said that women should avoid drinking completely if they are not using birth control and there is any chance they might be pregnant.
Guidelines in the United Kingdom also say that if you are pregnant or think you could become pregnant, the safest approach is not to drink alcohol at all.
On Friday, the National Health Service Greater Glasgow and Clyde launched a "No alcohol, no alcohol harm" campaign aimed at pregnant women about the risk of fetal alcohol spectrum disorder. There have been "mixed messages" around drinking while pregnant and the campaign aims to "put the issue to bed once and for all," according to the NHS.
But the new paper calls for more research on the effects of light drinking on pregnancy in order to better understand potential risks and to find answers to the questions many mothers-to-be might have.
For instance, "women often ask about 'safe' levels of drinking during pregnancy -- 'but one glass is OK, isn't it?'" Loubaba Mamluk, senior research associate in epidemiology at the University of Bristol in the UK and lead author of the paper, said in an email. "The distinction between light drinking and abstinence is indeed the point of most tension and confusion for health professionals and pregnant women."
"We were surprised that this very important topic was not researched as widely as expected," she said.
About one in 10 pregnant women in the United States, age 18 to 44, report that they've had at least one alcoholic beverage in the past 30 days while pregnant, according to the CDC.
What about just a sip?
The new paper included a systematic review and analysis of previous studies on low alcohol consumption and pregnancy that were published between 1950 and July 2016.
The researchers took a close look at studies that involved drinking up to 32 grams of alcohol per week, equivalent to about two pints of beer or two glasses of wine, Mamluk said. Out of thousands of studies, only 24 met the researchers' criteria for review.
"These were all representative studies of pregnant women or women trying to conceive who reported on their alcohol use before the baby was born," Mamluk said.
The researchers found that there was a dearth of evidence demonstrating a clear "safe limit" or "detrimental impact" of light alcohol consumption on a pregnancy, they wrote.
The scientific literature lacks data on low-level alcohol consumption during pregnancy, but the new paper did a pretty thorough job of trying to examine any possible sources of information, said Dr. Robyn Horsager-Boehrer, professor and chief of obstetrics and gynecology at the University of Texas Southwestern's William P. Clements Jr. University Hospital, who was not involved in the new study.
The most common alcohol-related questions that Horsager-Boehrer hears from pregnant patients involve concerns about a single drink they might have had before they knew they were pregnant or having a sip of champagne at a special event, she said.
"The question is really, 'What's the chance that if I just have this glass of champagne at my sister's wedding, is that going to be harmful?'" Horsager-Boehrer said, adding that her response is, "Nobody can quantify what that risk is. It is most likely low on the basis of the information we currently have, but you can't be promised that and you don't know that."
The new paper was "well done" and the conclusions were "appropriate," said Dr. Janet Williams, professor of pediatrics at the University of Texas Health San Antonio, who served as one of the lead authors on a 2015 American Academy of Pediatrics report advising no alcohol during pregnancy. She was not involved in the new paper.
Just because evidence for potential health risks associated with light drinking was not found in the new paper, that does not mean that there are no risks at all, she said.
"It is known that quantity and frequency of use, particularly binging, does correlate with increased risk," Williams said about fetal alcohol syndrome.
"Why not give the child the chance not to have this potential limitation or health risk in their life? There are so many other factors one can worry about, so how about one less concern? There are all sorts of non-risk-based beverages or ways to relax or express one's emotions that do not confer fetal or lifelong effects," she said.
On the other hand, for the mom-to-be who might have had a cocktail or two before she knew she was pregnant, as long as she was not binge drinking, there is little evidence to suggest that she should be overcome with worry, according to the paper.
The history of booze and pregnancy
While many health organizations strongly advise against drinking while pregnant -- the American Academy of Pediatrics says no amount should be considered safe -- some controversial studies previously have suggested that light drinking might not be harmful for the baby's future health.
A study that published in the Journal of Epidemiology and Community Health in 2010 found that children of mothers who drink one or two glasses of alcoholic beverages per week or per occasion during pregnancy were not at an increased risk of behavioral or cognitive problems by age 5.
In 2013, a study published in the journal BMJ Openfound no evidence to indicate that a mother's moderate alcohol consumption during pregnancy had an adverse effect on her child's ability to balance, a sign of neurological development, at age 10.
Many decades ago, oddly enough, it was common for doctors in Ireland to advise pregnant and nursing women to sip Guinness for what they thought could have been some health benefits, experts say.
Of course, until more research is conducted, many doctors now advise to avoid drinking alcohol during pregnancy -- no matter the type of boozy beverage.