Preterm birth is a national epidemic, costing the United States $26.2 billion each year, according to a recent report by the Institute of Medicine of the National Academy of Sciences—a price tag that's mostly the result of medical care for premature infants born with respiratory, gastrointestinal, immune, central nervous system and sensory problems.

And while most survive and catch up to their peers, babies who were born before 37 weeks of gestation are more likely to experience developmental problems such as learning disabilities, said Christine Dunkel-Schetter, PhD, a psychology professor at the University of California, Los Angeles.

"We look like an undeveloped country in terms of birth outcomes," said Dunkel-Schetter, at APA's 2009 Annual Convention. Nearly 13 percent of U.S. births are preterm, as compared with 5 percent to 9 percent for most other developed countries. "These are contributed to by the high rates of preterm birth among African-Americans and our high rates among whites of assisted reproductive technologies for treatment of infertility that result in multiple births."

In their work over the last two decades to pinpoint the causes of preterm labor, Dunkel-Schetter and her colleagues have found that stress—particularly in the form of worries and fears around pregnancy—is a strong predictor of preterm birth. One study by Dunkel-Schetter and her postdoctoral fellow, Roberta A. Mancuso, PhD, published in Psychosomatic Medicine (Vol. 66, No. 5), found that pregnant women who reported many pregnancy fears were more likely to give birth prematurely. More general sources of stress did not seem to have an independent effect on the timing of birth.

"I think we have pretty conclusive evidence that pregnancy anxiety, not stress in general, contributes to preterm labor."

Though researchers don't yet know what's unique about pregnancy-specific fears, they do have some sense of the mechanisms that cause preterm labor, said Dunkel-Schetter. In the Mancuso study, pregnancy anxiety predicted elevated levels of corticotropin-releasing hormone (CRH) in the mother's blood. This and other past research has found that maternal CRH levels rise naturally over the course of a pregnancy and directly affect fetal development and the timing of labor.

"Some people refer to CRH as the 'placental clock,'" she said.

In addition to stress's effects on a mother's hormonal system, it can also reduce her immune function, resulting in an increase of infections, which have been known to trigger preterm birth, said Dunkel-Schetter. When poorly managed, stress can also lead to unhealthy behaviors, such as exercising too little, or using cocaine, both of which have been linked to preterm birth, she added.

However, despite the many risk factors for preterm birth, "most babies are born on time and at a normal weight," said Dunkel-Schetter. Future research by Dunkel-Schetter's lab will aim to discover what factors protect future mothers from stress and its effects, she said.