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Aug. 20, 2004 -- Pregnant women who work the night shift are more likely to deliver their babies after their due dates, according to a new Danish study.
The new study looks at the effect of shift work on pregnancy length and birth weight of babies born to Danish women working outside the home from 1998-2001. The authors used data from more than 40,000 births in the Danish National Birth Cohort, the Central Population Register, National Patient Register, and National Birth Register.
The study focused on 32,000 women with daytime jobs, 1,000 women who worked evenings only, 400 women who worked nights only, and more than 6,000 women who rotated day, evening, and night shifts during the first six months of their pregnancies.
The researchers, led by Jin Liang Zhu, MSc, of Denmark's University of Aarhus, excluded women with more than one job. The study only focused on pregnancies where a single birth was expected; twins and triplets weren't part of the study.
The authors found that women who worked night shifts only had a higher risk delivering past their due dates and a higher risk of having babies who had low birth weight. Whereas women with shifting work schedules had "a slight excess" of babies who had low birth weight, write the researchers in the American Journal of Obstetrics and Gynecology.
Of the women with fixed nighttime hours, approximately one in six delivered past their due dates. According to the authors, these women had about a 35% higher risk of delivering past their due dates and an almost 80% risk of having babies with low birth weight.
The risk for late deliveries varied by occupation, write the authors.
"Night work may prolong the duration of pregnancy and reduce fetal growth, especially among industrial workers," write the researchers. "Industrial workers with fixed night work had a high risk of post-term birth."
The findings surprised the researchers.
"We expected rotating shift work with nights to have the highest risk of pre-term birth because of the changing nature of the work schedule and possible disruption of hormonal rhythm," they write. "However, that was not what we found."
Night work may affect the body's sleep-wake cycle enough to throw off its hormonal balance. "Hormone disturbances that are related to night work, and perhaps modified by the type of job, may be a mechanism for post-term birth," write the researchers, who call for further studies.
Other lifestyle factors could also be involved.
"Mothers with fixed evening or fixed night work were younger and more often smokers and industrial workers," write the researchers, who adjusted for occupation to take socioeconomic status into account.
SOURCE: Zhu, J., American Journal of Obstetrics and Gynecology, July 2004; vol 191: pp 285-291.