Standing for long periods during pregnancy may curb the growth of the developing fetus, suggests research published online in Occupational and Environmental Medicine.
Previous research has indicated that long working hours may increase the risk of birth defects, premature birth, stillbirth and low birthweight.
The researchers assessed the fetal growth rates of 4680 mums to be from early pregnancy onwards between 2002 and 2006.
Midway through their pregnancy, the women were quizzed about their work conditions and the physical demands of their jobs, including whether these included lifting, long periods of standing or walking, night shifts and long working hours.
Around four out of 10 (38.5%) of the women spent a long time on their feet and 45.5% had to walk for long periods. Heavy lifting was part of the job for just 6%, while around 4% worked night shifts.
The development of their babies was regularly measured throughout pregnancy, using ultrasound, and then again at birth.
The results showed that physically demanding work and long working hours were not consistently associated with restrictions on overall size or birthweight, or with premature birth.
And working up to 34 or 36 weeks of pregnancy had no adverse impact on fetal development.
But women who spent long periods on their feet during their pregnancy, in jobs such as sales, childcare, and teaching, had babies whose heads were an average of 1 cm (3%) smaller than average at birth, implying a slower growth rate.
Around half the women (47.5%) worked between 25 and 39 hours a week, while around one in four (23%) worked more than 40 hours a week.
And 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 1 cm smaller and a weight that was between 148 and 198 g smaller, on average, than babies born to women working under 25 hours a week. These differences were apparent from the third trimester (last three months of pregnancy) onwards.
The authors comment that generally women who are in work have fewer pregnancy complications, birth defects, and stillbirths than women who are unemployed, but that certain aspects of work may not be without risk.