Led by Jin Liang Zhu of the Danish Epidemiology Research Centre, Aarhus, the researchers used data from a Danish nationwide study to identify approximately 42,000 women who worked during pregnancy.
Rates of pregnancy loss were compared for women who worked during the daytime versus those who worked other shifts. Nearly 34,000 women worked daytime hours only; of those who worked other shifts, approximately 3,300 worked rotating shifts including night shifts. About 400 women worked fixed nighttime hours.
Overall, 1.5 percent of the pregnancies ended in fetal loss. Risk was no higher for women who worked rotating shifts -- whether or not they worked some night shifts -- compared to those who worked daytime hours. Working a fixed evening shift was also unrelated to risk of fetal loss.
However, risk of fetal loss was significantly elevated for women who worked a fixed night shift: 85 percent higher than for day workers, after adjustment for other factors.
The study also looked at the effects of job stress, which has previously been linked to certain health problems. Shift workers had more stressful jobs, and less control over their work, than day workers. However, job stress did not significantly affect the risk of fetal loss.
Previous studies have suggested that shift work may be related to increased risks of various health problems. Some studies have reported that shift work during pregnancy increases the risk of fetal loss, while others have found no relationship. The large Danish data base used for this study provided a unique scientific opportunity to examine the effects of work hours on pregnancy outcomes. The results show no effect of working rotating shifts or evening shifts on the risk of fetal loss. However, there does appear to be a significant increase in risk among women who work fixed night hours during pregnancy. The reasons for this relationship are unknown; increased estrogen levels related to night work might play a role.