Research indicates repeated heavy lifting or working with machinery on the job may increase a pregnant woman's chances of a pre-term delivery.

A study of nearly 3,000 pregnant women finds that for those having their first baby, the risk of premature rupture of membranes that protect the fetus (the amniotic sack) grew with the amount of fatigue women experienced at work. Findings were published in the February issue of the American Journal of Obstetrics and Gynecology.

Women not working outside the home had a two percent risk of premature rupture, versus more than seven percent for those with the most fatiguing jobs, according to the study.