UK researchers have found working up until the late stages of pregnancy can have as big an impact on a baby's health as mothers who smoke.

The research showed babies of mums who worked in their ninth month of pregnancy tended to be about 227g lighter than babies born to women who stopped working between six and eight months.

Researchers from the University of Essex looked at data from almost 31,000 mothers from two major UK studies and one US study.

Study author Professor Marco Francesconi told the UK's Daily Mail that low birth weight can lead to health problems.

"We know low birth weight is a predictor of many things that happen later, including lower chances of completing school successfully, lower wages and higher mortality," he said.

"We need to think seriously about parental leave, because –– as this study suggests –– the possible benefits of taking leave flexibly before the birth could be quite high."

The study showed the birth weight of babies born to working mums under the age of 24 was not affected, however older mothers who continued working appeared to have a bigger impact on their baby's birth weight.

Michael Permezel, a professor of obstetrics and gynaecology at the University of Melbourne, told ninemsn that overhauling maternity leave requirements is not necessary.

"There will be some women with more sedentary activities and many mothers actually say they do a lot less at work than when they are at home having to look after other children," he said.

"But on the other hand there are plenty of mothers who are too tired and need to stop."

Professor Permezel recommends women listen to their own bodies.

"The key message is to individualise — she should follow her body's message," he said.

(Source: http://health.ninemsn.com.au/healthnews/8507649/working-late-in-pregnancy-as-bad-as-smoking)