Expectant mothers who work up to their due date are likely to have babies with a lower birth weight, creating risks for the future, a study has revealed.
Women who worked after they were eight months pregnant had babies on average around half a pound lighter than those who stopped work between six and eight months.
The University of Essex research - which drew on data from three major studies, two in the UK and one in the US - found the effect of continuing to work during the late stages of pregnancy was equal to that of smoking while pregnant.
Babies whose mothers worked or smoked throughout pregnancy grew more slowly in the womb.
Past research has shown babies with low birth weights are at higher risks of poor health and slow development, and may suffer from multiple problems later in life.
Stopping work early in pregnancy was particularly beneficial for women with lower levels of education, the study found - suggesting that the effect of working during pregnancy was possibly more marked for those doing physically demanding work.
The birth weight of babies born to mothers under the age of 24 was not affected by them continuing to work, but in older mothers the effect was more significant.
The researchers identified 1,339 children whose mothers were part of the British Household Panel Survey, which was conducted between 1991 and 2005, and for whom data was available.
A further sample of 17,483 women who gave birth in 2000 or 2001 and who took part in the Millennium Cohort Study was also examined and showed similar results, along with 12,166 from the National Survey of Family Growth, relating to births in the US between the early 1970s and 1995.
The research, conducted by three economists, Prof Francesconi, Emilia Del Bono and John Ermisch, is published in the July edition of the Journal of Labour Economics, published by the University of Chicago.