Many mothers-to-be drink alcohol while expecting. There is little robust scientific evidence linking the occasional drink to health problems in children, but is one glass really safe?

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Many pregnant women wonder if light drinking during pregnancy is safe.

As I near the halfway point of my second pregnancy, a new study – led by Loubaba Mamluk, a senior research associate in epidemiology at University Hospital Bristol in the United Kingdom – addressed this very question. The findings of that study are published this week in the journal BMJ Open.

In a meta-analysis of previous studies, Mamluk and her colleagues looked at the risk of light drinking during pregnancy on birth complications and the baby’s health.

As Mamluk explained to me, “Women often ask about ‘safe’ levels of drinking during pregnancy – but one glass is OK, isn’t it?”

“The distinction between light drinking and abstinence is indeed the point of most tension and confusion for health professionals and pregnant women, and public health guidance varies worldwide,” she added.

The research team found an 8 percent increase in the risk of having a small baby and a 10 percent increase in the risk of preterm delivery in women who drank around two alcoholic drinks per week.

But there were no statistically significant effects on other factors, such as the baby’s long-term health.

However, a lack of evidence does not necessarily mean that moderate drinking is safe. To help lift the fog of confusion around this issue, I delved deeper. Here is what I found.

Most people are familiar with the concept that excessive alcohol consumption is bad for our health. But what does alcohol actually do?

I am reminded of the destructive effects of alcohol from my early days as a Ph.D. student. As part of a routine experiment, I sterilized small glass slides – which I later planned to add cells to – so I could look at them under the microscope.

After soaking the slides in a 70 percent alcohol solution for 15 minutes, I promptly removed the liquid – or so I had thought. Unbeknownst to me, a small amount was left in the petri dish. I then made the critical mistake of adding the cells straight away, exposing them to the leftover alcohol.

The next day, widespread death awaited me.

Similar to the way in which the cells in my experiment were bathed in alcohol, in a pregnant mother, alcohol can readily cross the placenta from the blood to the growing baby.

But the developing fetus cannot break down alcohol as quickly as a fully grown adult can. This means that while we may have cleared the alcohol from our system, the baby remains exposed to it for much longer.

Longer exposure means more time for alcohol to wreak havoc and kill cells.

When pregnant women drink heavily and regularly, it can cause a range of life-long issues affecting between 1 and 2 percent of children. This is called fetal alcohol spectrum disorder (FASD).

The American Congress of Obstetricians and Gynecologists (ACOG) define heavy drinking as “three or more drinks per occasion or more than seven drinks per week.”

FASD can affect children in many different ways. This is partly due to the fact that the speed with which alcohol is broken down varies from mother to mother, and the fact that alcohol exposure at different stages of pregnancy affects the developing baby differently.

The symptoms of FASD vary but can include an abnormal-looking face, poor learning and memory, hyperactivity, and difficulty in social situations.

While there is plenty of evidence linking heaving drinking to FASD, most pregnant women who do consume alcohol class themselves as low to moderate drinkers.

In the United States, 40 percent of pregnant mothers consume alcohol, of which between 3 and 5 percent report heavy drinking.

Looking across the globe, Irish women drink the most while pregnant (60 percent), closely followed by expectant mothers in Belarus (47 percent), Denmark (46 percent), and the U.K. (41 percent).

The National Institute on Alcohol Abuse and Alcoholism define one standard drink as 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of distilled spirit, such as vodka. One such drink contains around 14 grams of pure alcohol.

But these alcohol units are not the same in every country. In the U.K., for example, one standard alcoholic drink contains 8 grams of alcohol.

While this may not be important for mothers who spend most of their time in a single country, it is a nightmare for scientists. It makes it very difficult to compare studies on alcohol consumption conducted in different countries.

Undeterred, Mamluk and her colleagues collated the data from 26 high-quality international studies and used statistics to identify the overall risk associated with light alcohol consumption – which is around two standard U.S. drinks or four standard U.K. drinks per week.

The team looked at a range of possible complications during pregnancy, including miscarriage, stillbirth, gestational diabetes, and birth defects, as well as signs of FASD after birth. But they only found significant associations with an increased risk of small birth weight (8 percent) and preterm delivery (10 percent).

Mamluk explained to me that the lack of good-quality studies shows how difficult it is to design meaningful research into the effects of alcohol during pregnancy.

“It also illustrates,” she added, “the failure of researchers so far to focus on ‘light’ drinking versus no drinking, rather than just on moderate and heavy drinking – a question many expectant mums and healthcare providers care about.”

Commenting on the research, Prof. David Garry – who is the director of maternal fetal medicine at Stony Brook Medicine in New York – told me, “This type of research is extremely difficult to perform. It is unethical to attempt to have a group of women intentionally drink, so researchers have limited approaches to study alcohol effects.”

We hope that this most up-to-date evidence will help empower women to make an informed decision about drinking during pregnancy.”

Loubaba Mamluk

She continued, saying, “We emphasize that the evidence we have analyzed refers to light drinking, that the effects we 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.”

For Prof. Garry, who is a FASD Prevention Champion for the ACOG, total abstinence is the safest option. “It is clear that there is an association with alcohol consumption during pregnancy and [FASD],” he explained.

“It is also clear that harm from fetal alcohol exposure during pregnancy is completely avoided by not drinking during pregnancy.”

When I asked what advice Prof. Garry gives to his patients, he said, “I explain to potential pregnant women that drinking alcohol during pregnancy, including early pregnancy, can be harmful to their baby.”

“If you think you are pregnant you should not drink,” he added. “Women who are trying to become pregnant may not know if they are pregnant until several weeks after conception. You may wish to make a healthy choice and avoid drinking.”

But with nearly half of all pregnancies in the U.S. being unplanned, many of these women drink alcohol before they are aware that they are pregnant. Once they do know, feelings of stress and guilt are not uncommon.

From Prof. Garry’s point of view, a “[…] woman might miss her period but continue drinking with friends without considering she may be pregnant.” But in his experience, “many women assess their lifestyle and make changes including: stop smoking, avoid soda, take vitamins, and avoid alcohol.”

His message? “Avoiding alcohol can be considered one of the most healthy changes for a pregnant woman. No woman desires to harm their child. Using family and friends for support and positive reinforcement can help to reduce stress and avoid the harms of an alcohol-exposed pregnancy.”

A healthful lifestyle during pregnancy is, of course, the best way to get a new baby off to a good start and speed up recovery after birth.

Both the ACOG and the Centers for Disease Control and Prevention (CDC) echo Prof. Garry’s advice. They say, “There is no known safe amount of alcohol to drink while pregnant.”

But Mamluk and her colleagues say, “Women who have had a drink while pregnant should be reassured that they are unlikely to have caused their baby considerable harm, but if worried, they should discuss this with their [doctor] or midwife.”

The bottom line is that nobody is certain of the extent of harm that light drinking can cause a baby. In the absence of evidence, healthcare professionals and medical bodies err on the side of caution.

In the end, however, it comes down to personal choice. I made mine during my first pregnancy. It stood me in good stead, so I’m sticking with it this time around.