Standing for long periods of time and working long hours during pregnancy had a small negative effect on fetal growth, researchers found.
In longitudinal analyses, women who occasionally or often stood for long periods on the job gave birth to babies with slightly but significantly smaller head circumferences (P<0.05) when compared with babies of mothers who were not on their feet for long periods, according to Alex Burdorf, PhD, of Erasmus University Medical Center's Department of Public Health in Rotterdam, the Netherlands, and colleagues.
In addition, pregnant women who worked 25 hours a week or more gave birth to babies with smaller head circumference and lower weight (P<0.05) than those with shorter work weeks, the researchers reported online in Occupational and Environmental Medicine.
The differences were relatively small, however, with reductions in head circumference of about 1 cm (less than half an inch) and in birth weight of 148 to 198 grams (5 to 7 ounces).
"This study does not present concrete information on the required reduction in duration and level of work demands, which hampers sound advice," Burdorf and colleagues wrote. "The results of this study need to be confirmed by future research."
Some concerns have been raised that certain work-related factors -- long work hours, prolonged standing, psychological job strain, and physical demands -- could have negative effects on birth outcomes, but it remains unclear how the work load of pregnant women should be handled.
To explore the relationship between work factors, fetal growth, and birth outcomes, the researchers looked at data from the Generation R Study, a prospective cohort study tracking participants from birth to young adulthood.
The current analysis included 4,680 pregnant women (mean age 31) who gave birth between April 2002 and January 2006, completed a questionnaire about working conditions around 30 weeks of gestation, had paid employment, and delivered a live-born singleton.
Fetal growth was monitored repeatedly throughout the pregnancy using ultrasound.
Overall, 4.3% of babies were born with low birth weight (less than 2,500 grams or 5.5 pounds), 4.3% were born small for their gestational age, and 4.9% were born prematurely (before 37 weeks of gestation).
There were no consistent associations between physically demanding work or long working hours and any of those three adverse birth outcomes after multivariate adjustment. The main confounders were maternal age, prepregnancy weight, height at intake, parity, ethnicity, smoking, and folic acid use.
There were, however, relationships between certain working conditions and measures of fetal growth.
In a cross-sectional analysis, women who had long periods of standing on the job during their third trimester showed reduced fetal head circumference around 30 weeks of gestation -- a reduction of 0.80 mm (95% CI 0.14 to 1.46) for occasionally standing for long periods and 0.72 mm (95% CI 0.01 to 1.43) for often standing for long periods.
Long work hours were not associated with fetal growth in cross-sectional analyses.
But in a longitudinal analysis, compared with women working fewer than 25 hours a week, those working at least 25 hours a week showed lower growth rates for both fetal weight and head circumference.
"However, we must note that the population attributable fractions of specific categories of physically demanding work were very low, and the effects of fetal growth were subtle since these effects were not reflected in adverse birth outcomes," the authors wrote.
They acknowledged some limitations of the study, including the fact that mothers from ethnic minorities and with lower socioeconomic status were more likely to decline participation in the study and that women with paid employment generally have better pregnancy outcomes than women without a job.
In addition, the study did not include information on psychosocial stress or general fatigue and did not take into account other physically demanding activities outside of the paid job.
Snijder C, et al "Physically demanding work, fetal growth, and the risk of adverse birth outcomes. The Generation R study" Occup Environ Med 2012; DOI:10.1136/oemed-2011-100615.