General Obstetrics and Gynecology: Obstetrics
Shift work, duration of pregnancy, and birth weight:: The National Birth Cohort in Denmark

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Abstract

Objective

We used the Danish National Birth Cohort to estimate the effect of shift work on the duration of pregnancy and birth weight.

Study design

In the Danish National Birth Cohort (1998-2001), we identified women (and their singleton births): 32465 women with daytime work, 1038 women with fixed evening work, 400 women with fixed night work, 3137 women with rotating shift work (without night), and 3197 women with rotating shift work (with night) during pregnancy (first and second trimester). Birth outcomes were collected by linkage to the Central Population Register, the National Patient Register, and the National Birth Register. Gestational age at birth and birth weight were analyzed by means of general linear regression and logistic regression.

Results

Mean gestational age at birth and birth weight at term for daytime work were 281 days and 3616 g, although 281 days and 3586 to 3626 g for shift work. There were no statistically significant differences in gestational age at birth or birth weight at term between any types of shift work and daytime work. Fixed night work had a high risk of postterm birth (odds ratio, 1.35; 95% CI, 1.01-1.79); fixed evening work had a high risk of full-term low birth weight (odds ratio, 1.80; 95% CI, 1.10-2.94); and shift work as a group showed a slight excess of small-for-gestational-age babies (odds ratio, 1.09; 95% CI, 1.00-1.18). The risk of postterm birth was modified by maternal occupation. Industrial workers with fixed night work had a high risk of postterm birth.

Conclusion

Our results suggest that shift work during pregnancy had limited effects on indicators of fetal growth. Night work may prolong the duration of pregnancy and reduce fetal growth, especially among industrial workers.

Section snippets

Study population

From February 1998 to August 2001, 54,954 pregnant women (56,909 pregnancies) were enrolled in the Danish National Birth Cohort. The study methods have been described in detail elsewhere.14., 15. We identified 42,687 pregnancies for women who had a job and who stated their occupation and work schedule. If a woman provided >1 pregnancy to the cohort, we excluded all the subsequent pregnancies (n = 884). We found 41,150 pregnancies that resulted in live births by linking mothers to the Central

Results

The characteristics of the mothers and their babies are shown in Table I. Mothers with rotating shift work (with or without night) had similar age, prepregnancy body mass index, and life style as mothers with daytime work. Mothers with fixed evening or fixed night work were younger and more often smokers and industrial workers. The median number of night shifts was 14 per month for fixed night work, and 4 for rotating shift work (with night). The proportions of delivery by elective caesarean

Comment

Fixed night workers had a high risk of postterm birth, especially industrial workers; fixed evening workers had a high risk of full-term low birth weight; and women with shift work had a slight excess of small-for-gestational-age babies. The finding of a slightly elevated risk of small-for-gestational-age babies for shift workers has been supported by other studies.8., 9., 17. We expected rotating shift work that included night to carry the highest risk; however, this was not what we found.

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    Supported in part by grants from the Danish National Research Foundation and the Daloon Foundation (J.L.Z.); the establishment of the Danish National Birth Cohort has been financed by the Danish National Research Foundation, Danish Pharmaceutical Association, Ministry of Health, National Board of Health, Statens Serum Institut, BIOMED, March of Dimes Birth Defects Foundation, Danish Heart Association, Danish Medical Research Council, and Sygekassernes Helsefond.

    Reprints not available from the authors.

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