Previous research had indicated that long working hours might increase the risk of birth defects, premature birth, stillbirth and low birthweight.
The researchers assessed that the foetal growth rates of 4680 mothers 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 percent) of the women spent a long time on their feet and 45.5 percent had to walk for long periods. Heavy lifting was part of the job for just 6 percent, while around 4 percent 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 foetal 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 percent) smaller than average at birth, implying a slower growth rate.
Around half the women (47.5 percent) worked between 25 and 39 hours a week, while around one in four (23 percent) 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.
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.
The research was published online in Occupational and Environmental Medicine.
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