Pregnant women whose job is physically demanding - or who do shift work - could face more childbirth problems, say scientists.

A major study in the US showed that this heavy work was associated with premature delivery, high blood pressure and underweight babies.

Jobs which involved long periods of standing up, shift work and fatiguing work were also significantly linked with premature birth.

Physically demanding work was defined as work involving repetitive lifting, load-carrying or other significant physical exertion.

The study of 160,000 working women was carried out at the University of Michigan.

Dr Ellen Mozurkewich, who headed the research, said: "Activities that women engage in at work do affect their pregnancy outcome.

"Employers should either be willing to allow their pregnant employees to modify their work-related activities, or they should provide for longer leave time for pregnant women."

In the past 40 years, the number of working women in developed countries has nearly trebled, and women are on average working longer into their pregnancies.

Between 1961 and 1985, the number of US women working within one month of delivery more than doubled.

A rise in pregnancy complications has accompanied the trends. Between 1985 and 1996, the incidence of low birth weight babies in the US rose from 6.8% to 7.4%.

The study did not compare women who worked with those who did not work - it only compared women doing different sorts of work.

The study was published in the journal Obstetrics and Gynaecology.