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Mozurkewich, Ellen L. MD; Luke, Barbara ScD, MPH; Avni, Michal MPH; Wolf, Fredric M. PhD


Objective: To evaluate the association between working conditions and adverse pregnancy outcomes by performing a meta-analysis of published studies.

Data Sources: We searched the English-language literature in MEDLINE through August 1999 using the terms standing, posture, work, workload, working conditions, shift, occupational exposure, occupational diseases, lifting, pregnancy complications, pregnancy, small for gestational age (SGA), fetal growth retardation (FGR), preterm, and labor.

Methods of Study Selection: We included observational studies evaluating the effect of one or more of the following work-related exposures on adverse pregnancy outcome: physically demanding work, prolonged standing, long work hours, shift work, and cumulative work fatigue score. Outcomes of interest were preterm birth, hypertension or preeclampsia, and SGA.

We conducted a meta-analysis based on 160,988 women in 29 studies to evaluate the association of physically demanding work, prolonged standing, long working hours, shift work, and cumulative work fatigue score with preterm birth. Also analyzed were the associations of physically demanding work with hypertension or preeclampsia and SGA infants. The data were analyzed using the Peto-modified Mantel-Haenszel method to estimate the pooled odds ratios (ORs) and 95% confidence intervals (CIs).

Tabulation, Integration, and Results: Physically demanding work was significantly associated with preterm birth (OR 1.22, 95% CI 1.16, 1.29), SGA (OR 1.37, 95% CI 1.30, 1.44), and hypertension or preeclampsia (OR 1.60, 95% CI 1.30, 1.96). Other occupational exposures significantly associated with preterm birth included prolonged standing (OR 1.26, 95% CI 1.13, 1.40), shift and night work (OR 1.24, 95% CI 1.06, 1.46), and high cumulative work fatigue score (OR 1.63, 95% CI 1.33, 1.98). We found no significant association between long work hours and preterm birth (OR 1.03, 95% CI 0.92, 1.16).

Conclusion: Physically demanding work may significantly increase a woman's risk of adverse pregnancy outcome.

Women's roles in society have changed dramatically in recent decades. Women in the 1990s are more likely than their mothers to pursue formal education, work outside the home, marry later, and delay childbearing. During the past 4 decades, the number of employed women in the United States has nearly tripled, increasing from 23.2 million in 1960 to 62 million in 1996. This figure is projected to reach 70.3 million by 2005.1

In the United States, employed pregnant women also are working later into their pregnancies. Between 1961 and 1985, the proportion of women working during the last trimester of pregnancy increased by 50% (from 52% to 78%), whereas the proportion working within 1 month of delivery more than doubled (from 23% to 47%).2 This is in contrast to the trend in Europe and many other industrialized nations toward granting more liberal sick leave with pregnancy complications and more liberal maternity leave in general.

During this same period, there also have been major changes in the incidence of adverse pregnancy outcomes, with a recent reversal of improvements achieved during the 1960s and 1970s.3,4 Between 1970 and 1985, the incidence of low birth weight (LBW, less than 2500 g) declined steadily, but between 1985 and 1996, it increased from 6.8% to 7.4%, its highest level since 1978.5,6 Likewise, preterm births (before 37 weeks' gestation) rose from 8.8% in 1980 to 11% in 1996.5,6

Several observational studies found an association between working conditions and adverse pregnancy outcomes,7,8 whereas results of other studies seem to contradict such an association.9 We undertook this meta-analysis to evaluate the effect of working conditions on adverse pregnancy outcomes by combining results across the many observational studies in the literature.

A meta-analysis of 29 published studies indicates that physically demanding work is significantly associated with preterm birth, hypertension or preeclampsia, and small for gestational age birth weights.

Health Sciences Research, Divisions of Maternal-Fetal Medicine and Department of Obstetrics and Gynecology, University of Michigan Medical School, Ann Arbor, Michigan; and the Departments of Medical Education and Health Services, University of Washington Medical School, Seattle, Washington.

Address reprint requests to: Ellen L. Mozurkewich, MD, Department of Obstetrics and Gynecology, 1500 East Medical Drive, Mott Hospital F4385, Box 0264, Ann Arbor, MI 48109-0264, E-mail:

Received April 1, 1999. Received in revised form September 15, 1999. Accepted September 23, 1999.

© 2000 The American College of Obstetricians and Gynecologists