Doctors in Finland say they can tell from a baby's first poop if they're likely to end up overweight.
A specific group of bacteria are to blame, they say.
Dr Katja Korpela of Finland's University of Oulu led a team that looked at the first poop of 212 babies. Known as meconium, babies' first poops are made up of material they ingested while still in the womb.
The children's heights and weights were measured at age three, and those who were overweight were twice as likely to have had an abundance of Bacteroidetes phylum in their meconium three years' prior.
They also had a low amount of Proteobacteria.
"The microbiome of the first stool, formed in utero during fetal period, was associated with subsequent overweight at the age of three year," Dr Korpela told the European Congress of Clinical Microbiology & Infectious Diseases this week.
The bacterial composition of stools collected at one year of age provided no such insight, however.
The doctors also found babies who had a round of antibiotics in their first year still had lower populations by age three, "demonstrating the long-lasting impact of antibiotics on a child's microbiome".
Babies' microbiomes are currently being studied at the University of Auckland's Liggins Institute, specifically how C-section births affect them, and what long-lasting effects that might have.
The Finland study is yet to be peer-reviewed or published.