A new study shows that pregnant women who spend a lot of time on their feet -- and work more than 40 hours a week -- may give birth to smaller babies.
The study shows that women who spent long periods on their feet during their pregnancy in jobs such as sales, child care, and teaching had babies whose heads were an average of 1 centimeter smaller than average at birth.
Clocking long hours during pregnancy also had an effect on the baby's birth weight.
About half the women in the study worked between 25 and 39 hours a week. About one in four worked more than 40 hours a week.
The women who worked more than 40 hours a week had slightly smaller babies than those who worked less than 25 hours a week. But there were no differences in rates of preterm delivery, low birth weight, or babies being born too small for their gestational age, the study showed.
The differences in head size and weight were apparent from the third trimester onward.
Whether or not these differences affect a baby's long-term development is not known.
The findings appear online in Occupational and Environmental Medicine.
Protecting Pregnant Women at Work
The study suggests that changes are needed in work patterns for pregnant women.
"We believe that optimizing the work environment is important since participation of women in the reproductive age in the workforce continues to increase," the researchers write. The research team was led by Claudia A. Snijder, MD, of the Erasmus Medical Center in Rotterdam, the Netherlands.
The researchers say changes needed in the workplace include setting limits on:
- shift work
- night hours
These measures may be most effective if they focus on the third trimester.
Researchers used ultrasound to check the growth rates of the babies of 4,680 pregnant women in the Netherlands. The assessment was made throughout pregnancy and at birth.
Women also answered questions about their jobs, including whether they involved:
- long periods of standing or walking
- night shifts
- long hours
Close to 40% of women spent a long time on their feet and 45.5% had to walk for long periods as part of their job, according to the study. Just 6% of women engaged in on-the-job heavy lifting. About 4% worked night shifts.
More Frequent Ultrasounds May Be Warranted
"Anecdotally, we have always known that women who work harder have smaller babies," says Cynthia Gyamfi, MD. She is the director of perinatal clinics and an associate clinical professor of obstetrics and gynecology at New York-Presbyterian Hospital/Columbia University Medical Center in New York City.
Certain changes could help level the playing field for working moms and their kids, she says.
"We can modify the work schedule," she suggests. "We can also monitor growth more frequently with ultrasound among pregnant women who are on their feet a lot or who work long hours."
Ideally, more frequent monitoring would start in the third trimester. That's when these changes first pop up.
Ashley S. Roman, MD, says this study answers a question that she gets asked all the time: Is it safe to continue to work during pregnancy?Roman is a clinical assistant professor of obstetrics and gynecology at New York University Langone Medical Center in New York City.
"Many women don't have the option of either stopping work or cutting back on their work hours for financial reasons," she says in an email.
She points out that differences in the study did not emerge until late in pregnancy and working had no effect on preterm birth or low birth weight. "It is very reassuring that a large study such as this one with over 4,000 women included in the analysis found no harmful effect to working through most of pregnancy."