Stress during pregnancy can have long-term effects on gene expression, behavior, and brain functionality, but little is known about the effects on gut microbial composition and function. To find out, researchers recently examined the gut microbiome of wild macaques, whose gut microbiome and physiology resemble our own. Using gene sequencing, functionality simulations, and advanced statistical methods, Anzà and colleagues discovered that early gestation is a crucial period that affects gut microbial composition and function of young and adult macaques. High maternal glucocorticoids during early gestation translated into lower bacterial diversity, decreased anti-inflammatory species, and increased pro-inflammatory ones, with the effect being 10 times stronger when mothers experienced high stress during early gestation versus late gestation. These alterations became stronger with offspring age and were coupled with a functional reshaping that suggested the presence of inflammatory states, and increased susceptibility to diseases, including cancer. Such rapid “inflammaging” effects may be the result of developmental effects that originate before birth and might lead to similar inflammatory disorders and diseases in humans.