Strenuous or sedentary routine may raise preterm birth risk among poor
Low-income women who engage in strenuous physical activities as part of their daily routines during pregnancy are at increased risk of delivering preterm, according to a study of clinic patients in Maryland.l For instance, women who climb stairs more than 10 times a day are nearly twice as likely as women who climb stairs less frequently to deliver preterm. A sedentary lifestyle also increases one’s risk of preterm delivery, while exercising for leisure decreases the risk.
The analysis was based on women who received prenatal care between April 1988 and October 1989 at one of four clinics and were scheduled to deliver at the same urban hospital; women who gave birth at the hospital but had not received prenatal care were also included. During prenatal interviews, the researchers asked women about their substance use and physical activity during the first two trimesters; briefer postpartum interviews included similar questions regarding the third trimester. Additionally, information on the women’s pregnancy complications, obstetric history and prenatal care was obtained from the Maryland Perinatal Database. Using this information, the researchers identified women who had singleton births and whose infants weighed at least 500 g at birth; the resulting sample consisted of 1,172 women.
Most women in the study were aged 20-29 (55%), were black (67%), had had prenatal care (90%) and received public assistance (86%). Forty-five percent of the women smoked during their second trimester, and 10% used hard drugs at some time while pregnant. Few experienced complications during pregnancy or had a history of poor pregnancy outcomes.
Seventeen percent of the women delivered preterm (i.e., before 37 completed weeks’ gestation). The proportion delivering preterm was slightly above average among black women, smokers and women who received public assistance. It was substantially higher among those who had used drugs or who had had complications during pregnancy.
In bivariate analyses, women who climbed stairs more than 10 times a day were 1.5 times as likely to deliver preterm as were those who climbed stairs less frequently; women who walked for a purpose four or more days a week also had increased odds of delivering preterm (odds ratio, 2.2). Employment in a job that did not entail shift work and exercising during leisure time on 60 or more days throughout the first two trimesters were associated with significantly reduced odds of preterm delivery (0.5 for each). Women who watched fewer than 15 or more than 42 hours of television per week were more likely than other women to deliver preterm (1.7 and 2.8, respectively). Lifting heavy objects at home or at work, standing or moving on the job and amount of sleep per day did not have a significant effect on the chances that a woman would deliver preterm.
When the researchers conducted multiple logistic regression analyses to control for women’s background characteristics, obstetric history and use of tobacco and illegal drugs during pregnancy, climbing stairs more than 10 times a day remained a significant predictor of preterm delivery (odds ratio, 1.6), as did walking for a purpose four or more times a week (2.1). The effect of exercising 60 or more days during the first two trimesters was unchanged, and the odds of preterm birth associated with watching television fewer than 15 or more than 42 hours a week increased slightly (to 2.1 and 3.1, respectively). No other variables had significant effects in this analysis.
A final analysis controlling for women’s background characteristics and eliminating women who experienced complications during pregnancy confirmed the regression results. However, exercise during leisure time was no longer associated with reduced odds of preterm delivery.
The researchers observe that walking for a purpose and climbing stairs, as well as a sedentary lifestyle, may be correlated with low income and few resources. For example, women who walk four or more days a week may have no other way to get to work, and those who frequently climb stairs may live in housing projects; those who watch many hours of television a week may lack the resources needed to pursue other activities. For such women, the stressors of their living environment, rather than their level of physical activity alone, may contribute to the increased odds of preterm delivery. By contrast, women who exercise frequently may represent a select group who have more time for leisure activities and experience less stress than women who do not exercise. Thus, the researchers conclude that women’s necessary daily activities and daily stressors relating to poverty should be the focus of future studies of preterm delivery among low-income women.
1. Misra DP et al., Effects of physical activity on preterm birth, American Journal of Epidemiology, 1998, 147(7): 628-635.
Copyright The Alan Guttmacher Institute Nov/Dec 1998
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