New study details link between working conditions and problem pregnancies; Authors call for better maternity leave policies

March 30, 2000

ANN ARBOR, Mich. -- Physically demanding working conditions lead to a significant risk of adverse outcomes for pregnant women, demonstrating the need for a better national maternity leave policy, according to a new study by researchers in the University of Michigan Health System. Their study -- which comes amidst a growing national debate over maternity leave guidelines -- is published in the April issue of the journal Obstetrics and Gynecology.

The authors performed what is called a meta-analysis -- a large-scale study of previous studies. They examined 29 observational studies from 1966-99, involving more than 160,000 working women. The studies evaluated the relationship between work-related exposures and adverse pregnancy outcomes. Work-related exposures included physically demanding work, prolonged standing, long work hours, shift work, and work fatigue score. Physically demanding work was defined as heavy and/or repetitive lifting or load carrying, heavy labor or significant physical exertion.

Adverse outcomes measured were preterm birth (birth before 37 weeks gestation), hypertension or preclampsia, and SGA infants -- babies who are born small for gestational weight.

The authors found that physically demanding work was significantly associated with preterm birth, maternal hypertension and SGA infants. Prolonged standing, shift work and work fatigue score also were significantly associated with preterm birth. Long work hours, however, were not associated with preterm birth.

The authors conclude that physically demanding work may significantly increase a woman's risk of experiencing adverse pregnancy outcomes. Therefore, they say, these finding lend further support to calls for an improved national maternity leave policy for working women that includes pay, health benefits and job security.

"Activities that women engage in at work do affect their pregnancy outcome," says Ellen Mozurkewich, M.D., a fellow in the Division of Maternal-Fetal medicine in the U-M Health System, and principal investigator on the study. "Employers should either be willing to allow their pregnant employees to modify their work-related activities, or they should provide for longer leave time for pregnant women."

More than 120 countries provide paid maternity leave and health benefits by law. The United States is one of the very few industrialized countries that does not mandate paid maternity leave and health benefits. A recent congressional study found that, since maternity leave is unpaid, many women do not take full advantage of it due to financial concerns and fears over job security.

Mozurkewich says the issue is complicated by real world concerns.

"This is a major issue for obstetricians and working women who are pregnant," she says. "Although it seems very straightforward to modify the work environment for women who are pregnant, a lot of employers don't have a mechanism to do that."

In the past 40 years, the number of working women has nearly tripled. Additionally, employed pregnant women are now working later into their pregnancies. For example, between 1961 and 1985, the number of women working to within one month of delivery more than doubled. This comes amidst recent statistics showing a rise in adverse pregnancy outcomes. Between 1985 and 1996, the incidence of low birth weight rose from 6.8 percent to 7.4 percent, while preterm births rose from 8.8 percent in 1980 to 11 percent in 1996.

Mozurkewich says it's important to note that the studies analyzed in this paper used working pregnant women who did not engage in physically demanding labor as controls. She says this allows for comparison of women who engage in certain activities at work with those who do not engage in those activities -- it is not a comparison of women who work with those who don't.

Mozurkewich cautions that there are some limitations to this type of study. None of the studies they examined were interventional. There are currently no randomized studies in which working women are assigned either to continue physically demanding labor or to modify these activities. Unfortunately, she says, it would be extremely difficult and costly to actually perform an interventional study of pregnant women in the workplace.

In the absence of such a study, the authors say their findings dictate that physicians, employers and health-policy makers should pay serious attention to the relationship between workplace exposures and adverse pregnancy outcomes.

"This study provides the most current and comprehensive summary of the effect of specific aspects of employment during pregnancy on the course and outcome," says co-author Barbara Luke, Sc.D., MPH, professor of obstetrics and gynecology in the U-M Health System. "Assessing work factors is an important part of the prenatal care of working women, and modifying those shown to be detrimental can significantly improve outcomes."
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Mozurkewich and Luke's co-authors on the study are Michal Avni, MPH, U-M Department of Obstetrics and Gynecology, Division of Health Sciences Research, and Fredric M. Wolf, Ph.D., Departments of Medical Education and Health Services, University of Washington Medical School.

University of Michigan Health System

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