Working Night Shift, but Not Heavy Lifting, Is Risk Factor for Preterm Birth
By Katrina Woznicki,
Published: December 09, 2005
Reviewed by Robert Jasmer, MD; a__sistant Professor of Medicine, University of California, San Francisco .
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Inform pregnant patients who work jobs requiring hours of standing and/or heavy lifting that this study did not find an effect on the risk for preterm birth or reduced gestational size. However, this study found a significant increased risk between night-shift work and premature delivery.
HOUSTON, Dec. 8 - Pregnant women who work the graveyard shift face a significantly higher risk of preterm delivery, yet long hours, standing all day, and heavy lifting were not risk factors, investigators here reported.
Working between 10 p.m. and 7 a.m. during the first trimester raised the risk by at least 50% (relative risk 1.5, 95% confidence interval 1.0-2.0), according to Lisa A. Pompeii, Ph.D., of the University of Texas School of Public Health here.
The risk persisted into the second trimester (RR 1.6, 95% CI 1.0-2.3), the study of 1,908 pregnant women found, Dr. Pompeii and colleagues reported in the December issue of Obstetrics and Gynecology. And working at night during weeks 28 to 31 of pregnancy was a__sociated with a two-fold increased risk for preterm birth (odds ratio, 1.8, 95% CI 0.8-3.9).
Surprisingly, there were no significant a__sociations between preterm delivery and standing on the job for at least 30 hours a week or performing repeated heavy lifting, compared with women who stood six to 15 hours every week or those who did not lift, respectively. These activities were also not a__sociated with any increased or decreased risk in small-for-gestational-age births.
Interestingly, women who worked at least 46 hours per week during the first trimester showed a 40% risk reduction (RR 0.6, 95% CI 0.4-0.9), compared with women who worked 35 to 45 hours, regardless of the period of work exposure during their pregnancy.
These findings suggest several possibilities, wrote Dr. Pompeii and colleagues. First, night-shift work may interfere with the circadian rhythm and ultimately affect hormone levels, they speculated.
Secondly, women who worked longer hours also tended to have more education, were older, and were married. "An extended work week has been a__sociated with higher monetary income among U.S. workers, which is reflected in our study findings," they wrote. People from higher socioeconomic status tend to have better health outcomes.
The findings were based on data from the Pregnancy, Infection, and Nutrition study, a prospective cohort study that included 1,908 pregnant women who were recruited during prenatal visits that occurred from January 1995 to April 2000. The women were at 24 and 29 weeks gestation at the time of recruitment. Interviews were conducted both by telephone and then nested case-control participants, a total of 444 women, were interviewed in-person after delivery.
The participants were asked about their occupation, work schedules, and job duties. The women were also asked about smoking, v____al bleeding, and whether they vigorous leisure activities, such as exercise, during the first and second trimesters.
Preterm birth was defined as delivery before 37 weeks gestation. Fetal growth restriction was defined as infants whose birth weight fell below the 10th percentile.
Contrary to earlier beliefs, standing on the job all day and heavy lifting did not affect the risk for preterm delivery or reduced gestational size. It is possible, the authors said, that women who perform these kinds of jobs are already physically fit.
However, night-shift work remained a significant risk factor although the reasons weren't entirely clear.
The investigators said it is possible that working the graveyard shift may affect "nocturnal surges in uterine activity." Another theory is that the secretion of melatonin, involved in sleep cycles, may be suppressed in nighttime workers, which may also affect uterine activity.
The researchers also pointed out that few women in this study worked the night shift during their third trimester; 166 women reported night-shift work during their first trimester compared with 126 women in their second trimester and only 26 women in their final trimester. So the findings, they said, should be interpreted with caution.
Primary source: Obstetrics and Gynecology
Pompeii L. et al "Physical Exertion at Work and the Risk of Preterm Delivery and Small-for-Gestational Age Birth," Obstetrics & Gynecology, Dec. 2005; vol. 106, No. 6; p.1279-1288.