Less than 1 alcoholic drink a week in pregnancy is enough to change the fetal brain, study suggests

The findings suggest even occasional drinking can slow fetal brain development and alter the part of the brain that helps kids develop social skills, interpret sights and sounds, and understand language. 

While past research is clear that heavy drinking in pregnancy can lead to permanent and serious physical, cognitive, and behavioral problems in children, edicts that even light drinking is dangerous have come under more scrutiny by some doctors and parenting experts. 

The study authors told Insider their research is the first to use this type of technology to see exactly when and where alcohol exposure begins to affect the developing brain. 

And while not all, or even most, babies of pregnant drinkers will develop problems, the researchers say there's no guarantee the children won't, either. 

"It might be a very small risk associated with every glass you might drink during pregnancy, but you never know if that may be the one that pushes you over the edge," co-author Dr. Marlene Stuempflen told Insider. 

Researchers studied 24 brains out of an initial 500 to weed out confounding factors 

To conduct the study, doctors the Medical University of Vienna in Austria gathered a pool of 500 women who were receiving fetal MRIs for various clinical reasons. They then whittled that group down to the 51 who said they'd consumed some alcohol during pregnancy in an anonymous questionnaire. That's about 10%, which is in line with past estimates of how many pregnant people drink. 

(The researchers told Insider a psychologist was involved in the recruitment process, which emphasized making the environment a safe place for people to share honestly about their drinking habits.)

The doctors then eliminated all the moms-to-be whose fetuses may have had abnormal brain structures due to reasons other than alcohol, like cardiac conditions, genetic anomalies, or imaging errors. That left them with 24 fetal MRIs of drinkers to compare to fetal MRIs in non-drinkers at the same stage in pregnancy: between 22 and 36 weeks along. 

"We really put an emphasis on generating a very structured and very unbiased dataset and collection of patients," Stuempflen said. 

The study authors found that the fetal brains of drinkers were developing significantly slower than the brains of non-drinkers of the same gestational age. They also found that the right superior temporal sulcus, which plays a role in empathy, perspective-taking, language perception, and more, was shallower.

Specifically, the doctors noticed the fetal brains of drinkers were smoother and more symmetrical, whereas normally-developing brains have more folds and one hemisphere grows before the other.

"The most surprising thing to me was that fetuses who were exposed to a relatively low amount of alcohol developed this symmetrical brain," lead study author Dr. Patric Kienast told Insider. "That means that less than one drink a week, we already saw these effects." 

The study advances the group's work presented last year, which found that the fetal brains exposed to alcohol had a smaller paraventricular zone (the "birthplace" of all neurons, Stuempflen said) and a larger corpus collosum (the highway between brain hemispheres) than fetal brain that hadn't been exposed to alcohol.  

Since fetal alcohol spectrum disorder can manifest quite differently in all patients — from slight attention difficulties to noticeable facial deformities to learning disabilities and birth defects — it makes sense that alcohol seems to affect these wide-reaching brain structures rather than a single, contained region, Stuempflen said. 

Her team is planning follow-up research to see if and how these changes affect the children as they grow up. 

There's increasing evidence linking drinking in pregnancy with brain changes in the fetus  

The CDC, American College of Obstetricians and Gynecologists, and the American Pediatric Association contend there's no known safe amount of alcohol in pregnancy. The World Heart Federation goes as far as to say there's no safe amount of alcohol for anyone, pregnant or not. 

But statutes to eliminate alcohol entirely during pregnancy have been criticized as paternalistic and not evidence-based, since it's difficult to conduct high-quality studies on the harms of light alcohol consumption during pregnancy. 

Such studies must often rely on moms honestly recalling how much they drank many years prior, and can't tease apart all of the many factors — diet, exercise, healthcare access, stress, sleep, and social support, to name a few — that can affect a child's development. 

Past imaging studies have been in rats, or conducted retrospectively, like this 2020 paper finding that just one reported drink a week led to changes in the developing brain that can lead to behavioral deficits in children.

And there are studies that find no link between light or moderate alcohol consumption and developmental challenges in children. Parenting expert and economist Emily Oster has pointed to one Danish study, for instance, that found up to eight drinks a week during pregnancy has no effect on childrens' intelligence or attention levels. 

Kienast's team contends the potential risk of drinking, even if low, isn't worth it. "We know that prenatal alcohol exposure is the most important contributing factor to preventable cognitive impairments in children and, later on, adults," Stuempflen said.