Fetuses of mothers who show high rates of depression, anxiety and stress weigh less and are smaller than average at midterm, according to a recent study from the University of Miami School of Medicine.

Psychobiologist Miguel A. Diego and colleagues found that cortisol seems to be one potential mechanism for transmitting a mother’s stress to her unborn baby.

“Maternal distress is accompanied by biochemical changes, such as increased cortisol, that can both directly and indirectly affect the fetus,” Diego said. “Cortisol can directly cross through the placenta into the fetus, which could affect fetal development.”

The study, in the September-October issue of Psychosomatic Medicine, may shed light on previous findings that women with prenatal depression, anxiety or stress are more likely to deliver premature and low birth weight babies.

Cortisol “can also affect the mother’s vascular function, thereby reducing blood flow to the fetus, which could affect fetal growth by diminishing the amount of oxygen and nutrients that are delivered,” Diego said.

Ninety-eight women, between 16 and 29 weeks pregnant, took part in the study, which ran from September 1999 to January 2003. They were recruited from the prenatal clinic of the University of Miami Hospital and were predominantly of lower to lower-middle socioeconomic status.

The mothers completed questionnaires that measured overall levels of distress from daily hassles, depression and anxiety. Researchers used clinical ultrasound for fetal measurements and monitored levels of the hormones cortisol and norepinephrine in urine samples.

After analyzing the effects of demographics, maternal distress and hormonal levels, “prenatal cortisol was the only significant predictor of fetal weight,” the researchers found.

Diego said that the mother-fetal interaction is very well protected in most instances. It is only extreme levels of depression and anxiety that can affect the fetus.

“One of the things this research highlights is that if you are pregnant and under extreme amounts of stress or feeling depressed, you should talk with your doctor about ways of treating these conditions during pregnancy,” Diego said.

Larry Gray, a pediatrician at the University of Chicago Comer Children’s Hospital, said that Diego and his colleagues are at the frontier of trying to understand how a mother’s mental health, or state of mind, affects her baby. Gray was not involved with the study.

“I think this is a great study,” Gray said. “This association has been known clinically, but this is the first study where one has looked at a mother’s altered cortisol patterns before birth and how that affects the baby before it is born.”

This research was supported by a March of Dimes grant, two awards from the National Institutes of Health and funding from Johnson and Johnson Pediatric Institutes.

Psychosomatic Medicine is the official bimonthly peer-reviewed journal of the American Psychosomatic Society.