An international team of researchers found that the gut microbiomes of 2-year-olds reflect difficulties experienced by their mothers during their own childhood or pregnancy. This is the first research to document the intergenerational effects of adversity on the human gut microbiome.

The study, led by researchers at the University of California – Los Angeles (UCLA), builds on previous research showing that prenatal stress affects vaginal and gut microbiomes. As babies get their first gut microbes through the mother’s birth canal, the latter’s microbiomes are the foundation for her child’s, according to the UCLA news release. Previous studies have also revealed that the stress experienced by the baby in the womb and the psychological distress of the mother affect her microbiomes. The study was published in the Proceedings of the National Academy of Sciences.

In 450 mother-child pairs in Singapore, the UCLA study looked at the effects of abuse on mothers during their childhood, anxiety during pregnancy and their children’s exposure to stressful life events when they were two years old. The researchers also recorded any abuse or neglect the mothers experienced as children. The mothers’ main caregivers were also contacted to consider any abuse they suffered in the first two years of life.

The findings revealed that children whose mothers reported more anxiety during pregnancy had microbiomes in which microorganisms had populations of similar sizes, called “uniformity.” In addition, the microbiomes of children who experienced stressful life events after birth also had less genetic diversity, according to the statement.

An earlier study, in 2022, of wild geladas, a non-human primate living in Ethiopia, showed that the gut microbiome of a wild mammal exhibits clear and significant maternal effects both before and after weaning. According to this research, the effects of mothers on their child’s gut microbiome community can last long after the child has stopped breastfeeding, according to the Arizona State University news release.

“There are many questions about whether more diversity or uniformity is better or worse when the gut microbiome develops during infancy, so we don’t know if more is better at 2 years,” Francesca Querdasi, lead author of the new study, said in the UCLA statement.

Querdasi also said the findings indicated that the gut microbiome may be interacting with the immune system differently after adversity. The gut-brain microbiome connection develops rapidly during the first two to three years after birth, and changes due to adversity could likely influence children’s social-emotional development.