Standing for prolonged periods during pregnancy may curb the growth of the baby in the womb, research suggests.
Pregnant teachers, childcare workers and women that work in sales are among those whose babies are likely to have smaller heads when they are born - implying a slower growth rate.
The study, published online in Occupational and Environmental Medicine, examined 4,680 mothers-to-be from early pregnancy onwards.
The women were surveyed on their work conditions and the physical demands of their jobs. The progress of their babies was also monitored through ultrasound and birth measurements.
Just under two fifths of the women spent a long time on their feet at work. These women were likely to have babies whose heads were an average of 1cm smaller than the norm at birth.
Working for long hours also had an affect on fetal growth rate, the study found.
Of those surveyed, around half the women worked between 25 and 39 hours a week, while 23% worked more than 40 hours a week.
Those who worked more than 40 hours a week had smaller babies than those who worked under 25 hours a week.
Babies born to these women had a head circumference that was 1cm smaller and a weight that was between 148g and 198g lower on average than babies born to women working under 25 hours a week.
But working up to 36 weeks of pregnancy has no adverse impact on fetal development, researchers added.
"Long periods of standing and long working hours per week during pregnancy seem to negatively influence intrauterine growth," the authors conclude.
Gail Johnson, education and professional development adviser at the Royal College of Midwives, said: "This research provides a useful opportunity to discuss employment issues with women who are pregnant, and the midwife can help the woman to understand how pregnancy affects their body and the best way to stay healthy.
"Women need to be reassured that generally working in pregnancy does not increase the risk of poor outcomes. Whilst the finding suggest that there is some difference in the head circumference of babies born to women who stand for long periods, this does not seem to be associated with adverse outcomes.
"It is important for women to discuss with their employer any concerns they have around their jobs so that a solution to any problems - such as standing for long periods - can be found. I would also encourage women to seek advice from their midwife."