A study involving more than 40,000 Danish women found that shift work may have an effect on duration of pregnancy and birth weights of babies. Researchers studied women who worked during the day, evenings, nights, and those who worked rotating shifts. They found no statistically significant differences in the length of pregnancy or the birth weight of the baby among women when comparing all non-daytime shifts with those who worked only during the day. However, singling out night shift workers, they found a high risk of post-term birth; fixed evening workers had a high risk of full-term low birth weight; and shift workers as a group showed a slight excess of low-weight babies.

The researchers concluded, "Night work may prolong the duration of pregnancy and reduce fetal growth, especially among industrial workers." The study was published in the July issue of the American Journal of Obstetrics and Gynecology. For information on coping with shift work, see NSF's publication Sleep Strategies for Shift Workers.