New research says that pregnant women who work more than 32 hours a week in stressful jobs risk the health of their unborn child.

Researchers say that such babies have been found to be five ounces lighter than the average birth weight which equates with babies born to mothers who smoke.

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

The researchers say that mothers who work 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.

Professor Gouke Bonsel, who led the study, says women with high stress jobs should work no more than 24 hours from the beginning of pregnancy and he advocates the delegation or rescheduling of tasks to ensure the same amount of work is not done in a shorter time.

Professor Vivette Glover, from the foetal and neonatal research group at Imperial College London, agrees that more flexibility and understanding in the workplace was needed.

She says that stress increases the levels of a hormone called cortisol in the mother which can cross the placenta; cortisol is known to slow down growth and if a woman finds her job stressful, it could be damaging to the baby.

However, representatives of charities involved in promoting well being during pregnancy who agree that stress during pregnancy is undesirable, warn that a blanket rule would be unhelpful as many women have to work to survive and a one-size fits all provision in relation to working hours would not be effective.

They suggest women and employers should be encouraged to reach solutions that fit individual cases.

The Department for Trade and Industry says there are already protections, guidance and provisions in place to ensure the safety of working pregnant women and while women can go on maternity leave from 11 weeks before their baby is due, many choose to work into the eighth month in order to maximise their paid leave once the baby is born.