This increased risk seems to exist independently of other risk factors, such as a complicated first birth, young age and socioeconomic status, report researchers in the Aug. 9 issue of the British Medical Journal.

"It represents a true biological effect," says study author Dr. Gordon Smith, a professor of obstetrics and gynecology at Cambridge University in England.

Although the study does expand on certain aspects of the phenomenon, most of the news is not new. "It's long been known that short pregnancy intervals don't give the mother time to recover her own nutritional status," says Dr. Maureen Malee, director of maternal fetal medicine at the University of Miami School of Medicine.

A woman's body needs time to recover in other ways as well, even from a normal pregnancy. Many women are anemic after birth because they lose blood, and few mothers are getting the requisite eight hours of sleep a night plus three square meals a day. "Pregnancy and delivery are a huge phenomenon, never mind the phenomenon of trying to tackle the care of a newborn," Malee says.

Although recouping the mother's health is critical, other issues also play a part.

Social factors are also involved. In general, women whose interval between pregnancies was less than six months were more likely to be less than 20 years old, to smoke and to live in a poor area. "Most moms that are single, smoke and have a low socioeconomic status are not going to get prenatal care," says Dr. Jonathan Muraskas, director of neonatal perinatal medicine at Loyola University Medical Center in Chicago.

Smith and his colleagues wanted to try to sift out these independent factors, something previous studies had not done.

The research team excluded women who had a complicated first birth, concentrating only on those who had had a normal first birth. They used data from health records of 89,143 women in Scotland who had given birth a second time between 1992 and 1998. All of the women had conceived the second child within five years of the first.

Women who had conceived within six months of the first, normal birth were more than two times as likely to give birth to an extremely premature baby (24 to 32 weeks), 60 percent more likely to give birth to a "moderately preterm" birth (33 to 36 weeks), and the baby was more than 3› times more likely to die at birth.