New research suggests that babies born to mothers who experienced trauma during pregnancy—specifically the 9/11 attacks—may inherit altered stress responses due to changes in a key hormone: cortisol. Cortisol is commonly known as the body’s “stress hormone” and plays a major role in regulating blood pressure, blood sugar, and immune responses. But under extreme stress, the body’s cortisol system can become dysregulated—and this dysregulation may start before birth.What is cortisol and why does it matter?Cortisol is produced by the adrenal glands in response to stress. It helps the body manage challenging situations by raising blood glucose levels, increasing blood pressure, and aiding in immune function. Normally, cortisol levels rise when we’re under stress and return to normal afterward.However, in situations of severe or prolonged stress, the body may start using up cortisol faster than it can produce it. This can lead to abnormally low cortisol levels—a condition linked with fatigue, anxiety, and even immune system disorders.The 9/11 study: A natural experiment in traumaA unique study led by researchers from the University of Edinburgh and Mount Sinai School of Medicine focused on 38 women who witnessed the September 11, 2001 attacks on the World Trade Center. One year after the event, these women and their babies were tested for cortisol levels.The findings were surprising. Women who developed post-traumatic stress disorder (PTSD) following the attacks had significantly lower levels of cortisol—so did their babies. This suggests that the trauma experienced by the mother was, in some way, biologically passed on to the child.Timing matters: Third trimester appears most sensitiveInterestingly, the effect was strongest in women who were in their third trimester of pregnancy during the attacks. According to Jonathan Seckl of the University of Edinburgh, this detail points to a biological explanation rather than simply differences in parenting style. “That seems much more biological than about delivering care,” he said.This timing is crucial because during the third trimester, a baby’s brain and hormonal systems are undergoing rapid development. Any disruptions during this period—such as a sudden drop in cortisol levels—could “program” the baby’s stress response system for life.Nature, nurture, or both?While earlier studies linked maternal trauma to children’s stress disorders, many assumed this was due to storytelling or emotional modeling—children learning fear and stress from their parents’ accounts of traumatic events. But the findings from this study challenge that idea.Since the babies were only around one year old during testing, they were likely too young to have absorbed their mothers’ traumatic stories or behaviors in depth. Instead, the low cortisol levels may reflect biological programming in the womb or even shared genetic vulnerabilities to stress.Health risks associated with low cortisolExperts like Andrew Steptoe of University College London caution that while the connection between cortisol and stress is complex, low cortisol levels aren’t necessarily benign. “The most obvious [problem] is regulation of the immune system,” he noted. Chronic low cortisol can leave individuals more vulnerable to infections, inflammation, and mood disorders later in life.While more research is needed to fully understand how maternal trauma affects fetal development, the 9/11 study adds to growing evidence that stress during pregnancy can have long-term biological effects on children. These findings emphasize the importance of providing psychological support to expectant mothers—especially in times of crisis—to help buffer both them and their babies from lasting harm.