Researchers at the University Medical Centre in Rotterdam in the Netherlands who surveyed nearly 4,700 women with 30 weeks of pregnancy found that those who worked over 25 hours a week during pregnancy delivered babies which weighed about 200 grams less than average at birth. The average newborn weight is about 3.2 to 3.4 kg.
The circumference around their heads was nearly half an inch (1cm) shorter than the babies of non-working mothers, the researchers found.
They also found a connection between the amount of time pregnant women spent on their feet at work and the size of their babies, with those standing "often" in jobs such as teaching or sales more likely to have smaller children.
One explanation is that more physically demanding work may reduce the flow of blood to the placenta, limiting the amount of nutrients and oxygen going to the foetus.
Although it's unclear why working long hours in an office job would have a similar effect, it may be due to stress, they explained.
"We were not surprised that (the babies') head size was smaller in pregnant women who stand for a long time at work, but we were pleasantly surprised to find that it was only by a modest amount -- three per cent smaller than average at birth," lead researcher Prof Alex Burdorf was quoted as saying by the Daily Mail.
However, the researchers said that smaller babies are not necessarily unhealthier, but they may be at higher risk from breathing problems, heart defects and conditions affecting their digestion. Such babies may also suffer from learning difficulties and developmental problems later on, they said.
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In a paper published in the journal Occupational and Environmental Medicine, they said that their study suggests employers should do more to make pregnant women's jobs as comfortable as possible.
"Optimising the work environment is important since participation of women in the reproductive age in the workforce continues to increase," they said.
"Preventive measures reducing certain occupational conditions, such as shift work, night hours, standing, lifting and noise, have proven to reduce the risks of adverse birth outcomes."