New research shows pregnant women who work more than 40 hours a week often have smaller babies.
Dutch scientists found that the babies of harder working mums were 148-198 grams below the average birth weight of those who worked less than 25 hours a week.
They also found women who stand for long periods, in jobs such as childcare, teaching or sales, were more likely to have babies whose head circumference was one centimetre smaller than average.
However, working until 34-36 weeks of pregnancy had no impact on a baby's size.
The researchers from the University Medical Centre in Rotterdam in the Netherlands surveyed 4,680 women halfway through their pregnancy about how long they worked and how long they spent on their feet. The babies' size was measured throughout pregnancy and at birth.
Almost half of the women worked between 25 and 39 hours a week, while a quarter worked more than 40 hours each week.
"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 percent smaller than average at birth," said lead author Professor Alex Burdorf, whose study was published in the journal Occupational and Environmental Medicine.
The impact of size on a baby's health is still unknown, but the authors urge employers to cater working conditions to pregnant women.
"Optimising the work environment is important since participation of women in the reproductive age in the workforce continues to increase," the authors 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."
According to the UK's Daily Mail, there is some evidence that smaller babies are at higher risk of breathing problems, heart defects and digestive conditions. They could also be at risk of learning difficulties later in life.