Standing for long periods of time or working more than 40 hours a week while pregnant may affect the baby's development, Dutch researchers report.
In the new study, women who had jobs in sales, child care and teaching, which required spending many hours on their feet, had infants with heads about 3% smaller than women who worked in other jobs during their pregnancies, the researchers found.
Whether this makes a difference in the long-term development of the child isn't known, said lead researcher Alex Burdorf, from the department of public health at Erasmus University Medical Center in Rotterdam.
"We are not sure about that," he said. "But there are clear indications that a smaller head may negatively affect cognitive [brain] development."
Who needs to be concerned
Exactly how it might play a part in any one child's development isn't predictable, "but at a group level a smaller head is seen as a negative start," Burdorf said.
The only women who need to be concerned are those who stand all day and "whose doctor has indications that weight gain or fetalo growth is less than expected," he added.
The study established an association, and not a cause-and-effect link, between working conditions and baby size.
The report was published in the online edition of Occupational and Environmental Medicine.
How the study was done
For the study, Burdorf's team collected data on more than 4 600 pregnant women. The women were asked about their work situations including whether their jobs required lifting, standing, walking, long hours or night work.
The researchers measured the development of the babies throughout the pregnancy and after birth. They found that physically demanding work had no effect on the infant's size, weight or whether the child was premature.
There was also no effect on infants of mothers who worked right up to the month before giving birth.
Women who spent a lot of time standing, however, had infants with an average head size 1 centimetre smaller than infants of women who didn't spend a lot of work time on their feet.
In addition, women who worked more than 40 hours a week were more likely to give birth to infants with smaller heads that weighed less than infants of women who worked less than 25 hours a week, the researchers found.
These findings may mean that standing and working very long hours has a negative effect on the infant's development, Burdorf's group said.
Work good during pregnancy
Apart from these exceptions, work is generally a good thing during pregnancy, Burdorf's team noted. Women who work have pregnancies with fewer complications and have fewer stillbirths or infants with birth defects, compared with women who don't work, they said.
Dr. Jill Rabin, chief of ambulatory care, obstetrics and gynaecology at Long Island Jewish Medical Center in New Hyde Park, N.Y. was sceptical about the findings.
"The study poses more questions than it answers," she said.
One problem with the study is that all the data were self-reported so there is a possibility the data is not completely accurate.
In addition, while the researchers took into account some other factors, such as drinking and smoking, height and weight, they didn't account for others. Particularly important are diet and previous pregnancies, Rabin said. "These factors may be the most important," she noted.
It is also not possible to know from the study if this small difference in head size will have long-term consequences.
"Whether or not these infants will have long-term effects can only be determined by following them over time," she said.
All about pregnancy
For more information on having a healthy pregnancy, visit the U.S. Department of Health and Human Services Office on Women's Health.
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