BOSTON, OCT. 10 -- Working long hours at a stressful job does not pose significantly greater risks to a successful pregnancy among otherwise healthy women of high socioeconomic status, according to a new study.

The study, which appears in Thursday's New England Journal of Medicine, challenges previous research suggesting that women in physically arduous occupations face a greater likelihood their pregnancies will result in miscarriage or stillbirth.

A common criticism of the earlier studies has been that women with physically demanding jobs are likely to be of lower socioeconomic status, who often have less access to medical care and have poorer health habits.

To address that distinction a research team led by Mark A. Klebanoff of the National Institutes of Health surveyed 989 female medical school graduates who gave birth during residency, a prolonged period of intense training at a hospital.

The residents on average worked 70 hours per week and were subject to severe emotional stress -- working conditions that might be expected to cause a high risk of premature birth, undersized babies and high blood pressure during pregnancy.

Klebanoff and his colleagues compared the women doctors to 1,238 physicians' wives who gave birth while their husbands were in residency. When they worked, the doctor's wives tended to put in half as many hours as the female physicians.

The researchers found that both groups of women had virtually the same number of miscarriages, tubal pregnancies, stillbirths, premature births and underweight babies.

"These results suggest that working long hours in a stressful occupation has little effect on the outcome of pregnancy in an otherwise healthy population of high socioeconomic status," the Klebanoff team concluded.

Only when the women doctors worked more than 100 hours per week did they seem to have a higher risk of premature birth.