After the eighth week of pregnancy, women working two or more shifts per week have a 32 per cent higher risk of miscarriage compared with women who not working night shifts.

The risk of miscarriage increased with the number of night shifts worked per week and also by numbers of consecutive shifts.

The authors say women working at night are exposed to artificial light, which disrupts their circadian rhythm and decreases the release of melatonin.

The hormone regulates sleep-wake cycles and has been shown to be important in maintaining a successful pregnancy, possibly by preserving the function of the placenta, the report said.

However, they said the study is observational and can't establish the cause of the miscarriages.

About 14 per cent of women in Europe work at night at least once a month and the findings could have ramifications for workplace laws.

"The findings increase the knowledge about exposure to night work and have relevance for working pregnant women as well as their employers, physicians and midwives," the report said.

The first of its kind study matched payroll data for more than 20,000 women employed at Danish public hospitals with the country's national medical records for births and miscarriages.

The study was published online in Occupational & Environmental Medicine on Tuesday.

Australian Associated Press