Volume 33, Number 3, May/June 2001
Job-Related Fatigue During First Pregnancy May Cause Early Membrane Rupture
Working women pregnant with their first child who report occupational fatigue run a greater risk of preterm premature rupture of membranes than their counterparts who do not work outside the home or who work outside the home but do not experience occupational fatigue.1 The risk of preterm premature rupture of membranes increases significantly as the number of sources of occupational fatigue increases--from 2% among women not working outside the home to 7% among those who report four or five sources of fatigue on their job. In addition, the number of hours worked per week is significantly associated with preterm premature rupture of membranes in working women pregnant with their first child. Similar associations are not apparent among working women who have given birth previously.
To determine the relationship between occupational fatigue and spontaneous preterm birth (subdivided into spontaneous preterm labor, preterm premature rupture of membranes and indicated preterm delivery), researchers analyzed data from the Preterm Prediction Study. The prospective study, which was conducted at 10 locations between October 1992 and July 1994, included women who were 22-24 weeks pregnant at enrollment and had a singleton gestation and intact membranes.
Researchers obtained detailed socioeconomic, medical and obstetric data from each woman through interviews and by reviewing medical records. They also asked each participant to complete a nurse- administered questionnaire about her current employment, the number of hours she worked per week and sources of occupational fatigue. The questionnaire asked about five specific sources of job- related fatigue: posture (standing for more than three hours daily); work with industrial machines; physical exertion; mental stress (doing repetitive or boring work); and working-environment stress (working in a cold, wet or noisy area). Participants were followed up until they delivered, and outcome data were collected.
The analyses include data on 2,929 women--1,218 who had not given birth before (nulliparous women) and 1,711 who had had at least one previous birth (multiparous women). Participants were predominantly black and low-income (62-63%); about one-third had less than 12 years' schooling. More than half had symptoms that suggested preterm labor, three in 10 smoked during pregnancy and one-quarter had vaginal bleeding within the first two trimesters. Overall, 14% of the women delivered preterm (before 37 weeks' gestation). Preterm premature rupture of membranes occurred in 5% of pregnancies and accounted for 33% of all deliveries before 37 weeks' gestation.
In univariate analyses, preterm premature rupture of membranes was linked to each of the five sources of occupational fatigue for nulliparous women; relative risks, when these women were compared with their counterparts who did not work outside the home, ranged from 2.6 to 3.1. Furthermore, the absolute risk grew significantly as the number of sources of occupational fatigue increased: from 2% among women not working outside the home to 7% among those reporting 4-5 sources of fatigue. No other category of preterm delivery was associated with job-related fatigue among women pregnant with their first child. However, these women also showed a statistical link between preterm premature rupture of membranes and the number of hours worked per week: The risk of this outcome ranged from 2% for women not working outside the home to 9% for women working more than 40 hours per week.
Results of multivariate analyses that took into account women's socioeconomic background and clinical characteristics confirmed that nulliparous women who reported job-related fatigue had a significantly elevated risk of experiencing preterm premature rupture of membranes. The risk was more than doubled for those who worked with industrial machines (odds ratio, 2.2) and was nearly doubled for those reporting other sources of fatigue (1.6-1.7).
Occupational fatigue was not associated with any of the categories of spontaneous preterm delivery for multiparous women at the univariate level and therefore was not examined in multivariate analyses. The researchers initially speculated that this lack of association might reflect that women who have previously borne children preterm might limit their work activities during subsequent pregnancies. However, they found no significant demographic differences between women with and those without a history of preterm delivery, and the same proportion of women in both groups (41%) worked during the study pregnancy.
According to the researchers, the major strength of their study is that it considers the individual components of occupational fatigue in relation to the various categories of spontaneous preterm delivery. They acknowledge, however, that its results may not be generalizable to the general obstetric population and that the findings are limited by deficiencies in the questions asked.
Commenting on their findings, the researchers observe that it remains to be seen why occupational fatigue is associated with an increased risk of preterm premature rupture of membranes only among nulliparous women. Given the "obvious public health importance" of such an association, they conclude that "studies to determine which nulliparous women may be at risk and why multiparous women appear able to avoid this risk need to be designed."--J. Ochs
1. Newman RB et al., Occupational fatigue and preterm premature rupture of membranes, American Journal of Obstetrics and Gynecology, 2001, 184(3):438-446.