Gwyneth Paltrow, who gave birth in April

London, May 28: Pregnant women who work more than 32 hours a week in stressful jobs risk the health of their unborn child, new research claims.

Babies born to these women have been found to be five ounces lighter than the average birth weight ? the same as those of pregnant smokers.

The results provide fresh evidence of the effect of stress on a developing baby and has led the man in charge of the research to call on pregnant women to work no more than 24 hours a week.

The study, involving 7,000 women, and conducted by the Amsterdam Born Children and their Development research group, also found that mothers suffering from stress are more likely to have babies that cry excessively.

And it revealed that mothers who worked long hours have an increased risk of developing pre-eclampsia, a serious complication of pregnancy caused by a defect in the placenta that restricts blood flow to the baby.

Prof. Gouke Bonsel, who headed the study, said: “Women with high stress jobs would do better to work no more than 24 hours from the beginning of pregnancy.”

To ensure that they didn’t simply end up doing the same amount of work in a shorter time, he advised they should delegate or reschedule tasks.

Responding to the research, Prof. Vivette Glover, from the foetal and neonatal research group at Imperial College, London, said more flexibility and understanding in the workplace was needed.

“Stress increases the levels of a hormone called cortisol in the mother. This hormone can cross the placenta and we know that cortisol can cause a slowing in growth,” she said. “The effects on an unborn child all depends on the woman’s attitude to the work. If they find the job stressful, it could be damaging to the baby.”

Nick Morgan, a spokesman from Tommy’s, the baby charity, said women should try to reduce their stress levels during pregnancy, but should have the freedom to choose the best way this can be done.

“Whether a blanket reduction in working hours is the best way to do that may be another matter,” said Morgan. “Stress is very subjective. Some women could become more stressed if they are only able to work for 24 hours each week.”

A spokesman for the Department for Trade and Industry said: “As women work in many different roles, a one-size fits all provision in relation to working hours would not be effective. There are protections, guidance and provisions in place to ensure the safety of working pregnant women.”