Psychology professor Elysia Davis and her colleagues — including undergraduate and graduate students — have spent years studying the effect pregnant women’s stress reactions have on the fetus. Photo: Wayne Armstrong

This article first appeared in the Newsweek Special Edition “Your Baby’s Brain: How New Science is Unlocking the Secrets of the Infant Mind.”

Don’t eat sushi. Don’t drink alcohol. Ditto for coffee.

Steer clear of hot tubs. Your hormones may make you feel like you’re on a metaphorical roller coaster, but you should not get on a literal one.

Pregnant women have grown used to enduring the ever-lengthening list of things they need to avoid to ensure a safe and healthy birth. A new University of Denver study suggests that stress — one of the most natural reactions a mother can have while walking around with a tiny human inside her womb — can be harmful to babies, possibly wiring the fetal brains for worry and anxiety permanently.

Psychology professor Elysia Davis and her colleagues — including undergraduate and graduate students — have spent years studying the effect pregnant women’s stress reactions have on the fetus. One hormone Davis has focused on is cortisol, which the body produces and passes through the placenta to the unborn child.

“Cortisol plays an important role in regulating the maturation of the fetus, such as lung development,” Davis says. “These stress systems in the body aren’t just there to cause damage or harm us.”

A pregnant woman can expect her cortisol level to increase by two to four times. But when Davis and her colleagues studied expectant mothers with cortisol levels consistently higher than normal early in the pregnancy, they made a startling discovery: Their infants displayed a much higher sensitivity to stress than other babies.

“After birth, every baby gets its blood drawn by the hospital,” Davis says. “The babies who had the largest cortisol response to the stress of having their blood drawn were the ones whose mothers had high levels of cortisol when they were pregnant. The babies exposed to higher maternal cortisol before birth had a bigger stress reaction.”

As these babies grew from infancy to toddlerhood, they exhibited heightened levels of anxiety compared to other children. “In our lab, we presented the 2-year-olds a series of challenges, such as exploring a room with toys including a floor-level balance beam or having a stranger enter the room and roll a ball to them,” Davis says. “These toddlers consistently showed more fearful behaviors in response to these moderately challenging situations.”

Instead of enthusiastically engaging in play with the stranger with the ball, for instance, the toddlers would instead stand frozen by the wall or run back to their mothers. “They showed more fearful expressions than normal,” Davis says.

By the time the children were between ages 6 and 9, MRI scans revealed that the children’s amygdala, the section of the brain associated with the human response to frightening stimuli, were larger than normal. “Not only are mothers reporting that the children show anxious behavior, such as being scared of going to school, but we can see differences in the way their brains have developed,” Davis says.

The next step for Davis and her colleagues is to observe the children as they enter adolescence, when many mental health disorders, such as depression, begin to emerge.

The study highlights the importance of keeping both a healthy body and a sound mind for expectant mothers.

“We don’t pay attention to women’s mental health during pregnancy the same way we do to other areas, such as their nutrition,” Davis says. “We know that women with strong social support, whether from their partner or their sister or their friends, are protected.”

The best advice Davis can give worried pregnant mothers is to take care of their mental health by carrying on with the activities they normally enjoy.

“Whether it’s going out for a walk or hanging out with friends, we all know what we can do to take care of our mental health,” she says.