Gut Reaction

Study implicates bacteria in maternal stress and infant illnesses

Pregnant women in Sri Lanka (Photo by Peter van der Sluijs)
Pregnant women in Sri Lanka (Photo by Peter van der Sluijs)


Pregnancy, it is often assumed, is a happy time; Mom is “glowing” with new life growing inside her. But some 13 percent of pregnant women and new mothers don’t feel glow-y at all: they are in the throes of depression, an illness that may be exacerbated during pregnancy by hormonal changes that affect brain regions controlling emotion and mood.

Maternal stress and depression during pregnancy can affect newborns as well: studies show that stress is correlated with preterm birth and low-birth weight, as well as asthma, eczema, anxiety, and symptoms of attention-deficit-hyperactivity disorder (ADHD) in children.

The mechanisms behind these associations in infants have been mysterious, but a new study of pregnant women in the Netherlands links prenatal stress to changes in the infants’ gut bacteria—and to a higher incidence of intestinal problems and allergies in their children.

“There is a large body of literature from the last 15 years [or so] that shows that stress and anxiety during pregnancy, even in healthy, well-fed, otherwise ‘normal’ populations, is associated with all types of less desirable outcomes in the offspring,” the study’s lead author, Carolina de Weerth, wrote to me via email. While the results are “correlational and not causal,” there is enough evidence, she says, that moderately high levels of psychological distress in the mother can affect fetal development. Her study, she believes, is the first “to show a possible novel pathway through which maternal psychological stress” may influence development of a baby by affecting the infant’s “microbial colonization.”

De Weerth and Maartje Zijlmans, developmental psychologists at Radboud University Nijmegen, and molecular biologist Katri Korpela and colleagues at the University of Helsinki, followed 56 health full-term, vaginally delivered infants for 110 days after birth. During the third trimester of pregnancy, the mothers filled out questionnaires assessing anxiety levels, including the fear of giving birth to a handicapped child. Around the 37th week of pregnancy, the mothers gave saliva samples to the researchers, which they measured for concentrations of cortisol, a stress hormone. Following birth, infant fecal samples were analyzed for the presence of different groups of bacteria. The mothers were also asked to note if their babies had symptoms of allergy or gastrointestinal illness.

The babies whose mothers had experienced high levels of stress had significantly higher levels of bacterial groups known to contain infectious pathogens and lower levels of “good” bacteria. The babies born to higher-stress mothers also had more allergy symptoms and gastrointestinal distress.

De Weerth and her colleagues offer several hypotheses to explain these changes, including the possibility that cortisol from the mother can cross the placenta, increasing the permeability of the gut wall or targeting immune cells, and thereby shaping the communities of bacteria that can grow there. It’s also possible that the bacteria themselves travel via the bloodstream from mother to child in utero, seeding the amniotic fluid, which the fetus swallows.

This pathway is “extremely relevant,” De Weerth says, “because it may be modified in a relatively easy way.” For example, it may be possible to change the composition of the infant’s gut bacteria by giving women good bacteria, in the form of probiotics, during pregnancy. One study in human adults has even shown that taking probiotics may reduce stress, depression, and anger or hostility.

“Given all the evidence we have to date, I think we certainly should do more prenatal screening of psychological distress in pregnant women,” De Weerth says. “Over the last century we have made enormous advances in the medical care of mother and fetus, and have drastically reduced the age of survival of preterm children. But care for the psychological state of the mother has remained behind. … Women that are stressed and anxious should be cared for just as much as women with higher blood pressure or blood sugar are cared for during pregnancy.”


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Josie Glausiusz writes about science and the environment for magazines that include Nature, National Geographic, Scientific American Mind, Discover, New Scientist, and Wired. From 2013 to 2015 she wrote The American Scholar’s “On Science” blog. Her Hakai Magazine article, “Land Divided, Coast United,” won Amnesty International Canada's 2015 Online Media Award.


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