For years, researchers have documented health differences between babies born vaginally versus by cesarean section—work that revealed the importance of exposure to the vaginal microbiome during birth. But a new Dartmouth-led study of more than 3,500 children in Brazil suggests another critical factor may have been overlooked: labor itself.
In a new study published in the Proceedings of the National Academy of Sciences, Dartmouth researchers found that children born via C-section before labor had about 6–7% lower levels of hair cortisol at ages 4 to 7 compared to those born vaginally or by cesarean after labor had started.
This distinction between planned and unplanned cesareans—which differ in whether the baby experiences labor—points to a potentially significant developmental exposure that extends well beyond the delivery room.
Professor Zaneta Thayer ’08
Cortisol, often called the “stress hormone,” plays a critical role in regulating growth, metabolism, immune function, and behavior. Hair cortisol offers a window into systemic cortisol levels over several months, making it a powerful tool for large-scale studies. These differences were statistically significant and persisted across middle childhood (ages 4 and 6–7 years).
“The paper explores the potential developmental significance of labor—which we know is long and physiologically stressful for humans—on children’s longer-term biology,” says Associate Professor of Anthropology Zaneta Thayer ’08, the study’s lead author.
Human labor is not just a mechanical process; it’s a cascade of hormonal and physical stimuli shaped by evolutionary pressures. During labor, babies experience rhythmic contractions and surges of hormones like cortisol and oxytocin—signals that help prepare them for life outside the womb. Planned cesarean births bypass these cues entirely.
“Children born through cesarean before labor, as occurs in planned cesarean, have lower hair cortisol levels at 4–7 years of age compared to children born after labor, whether via vaginal delivery or unplanned cesarean section,” Thayer says. “These effects are particularly pronounced when children are born without labor after a shorter gestation length.”
There was no difference between the vaginal delivery and the C-section-after-labor groups, suggesting that exposure to labor, rather than surgical delivery, drives this difference.
The significance of these lower cortisol levels remains unclear, but patterns from other research offer clues. Lower cortisol in childhood has been linked in other studies to early life trauma, and conditions like attention deficit hyperactivity disorder.
Preterm delivery has also been associated with lower childhood cortisol. In this study, Thayer found that children born after a shorter gestation length also had significantly lower cortisol, with these effects most pronounced if the child was born via C-section before labor. The difference in cortisol between children born vaginally and through C-section before labor attenuated if the children were born after 39.5 weeks.
Brazil provided an ideal setting for this research because it has the second highest prevalence of C-sections globally. Previous research indicates that the majority of these surgeries are elective rather than medically indicated, therefore reducing confounding factors related to maternal and child health that could also impact cortisol. A separate analysis that matched participants on clinical risk factors that would lead to planned cesarean, including maternal age, parity, high blood pressure, and diabetes, replicated these results. The findings raise important questions for clinical care worldwide.
“There are many implications for clinical care, including a consideration of the potential benefits of labor for individuals considering elective cesarean and potentially waiting later in gestation for a planned cesarean,” Thayer says.
The team plans to follow these children over time to see whether lower cortisol mediates other health outcomes previously linked to cesarean delivery, such as obesity and behavioral disorders. Future research may even explore interventions that mimic labor’s hormonal or mechanical signals for babies born via planned cesarean.
The research adds important context to ongoing discussions about the rising global rates of cesarean deliveries and the potential trade-offs involved in elective procedures.
“Labor could be an important developmental step shaped by evolution,” Thayer says. “Bypassing it could have unintended biological consequences.”