Working two or more night shifts in a week may significantly increase a pregnant woman's risk of miscarriage, a study has found.

Previous studies have suggested that pregnant women face a greater risk of miscarriage if they work night shifts, but they have been based on self-reported shift work and have not quantified the level of increased risk or the amount of shift work involved.

Researchers from the University of Copenhagen and Aarhus University Hospital in Denmark accessed payroll data on 22,744 pregnant women working in public services, mainly hospitals.

They linked that with data from Danish registers on births and admissions to hospital for the miscarriage to determine how the risk of miscarriage between weeks 4-22 of pregnancy was influenced by night work.

Overall 377,896 pregnancy weeks were included -- an average of 19.7 weeks per woman, according to a study published in the journal Occupational & Environmental Medicine.

After week eight of pregnancy, women who had worked two or more night shifts the previous week had a 32 per cent higher risk of miscarriage compared with women who had not worked any night shifts that week.

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

The association between night work and the risk of miscarriages was stronger after pregnancy in week 8.

"This may be explained by the decline in the proportion of chromosomally abnormal fetuses with gestational age, which makes an association with environmental exposure more easily detectable among later miscarriages," researchers said.

This is an observational study, and as such, cannot establish cause, and researchers point out that data on miscarriages, particularly early miscarriages, were incomplete.

However, as around 14 per cent of women in Europe report working at night at least once a month, the findings have relevance for working pregnant women as well as their employers, physicians and midwives, they said.

"Moreover, the results could have implications for national occupational health regulations," they said.

In terms of the underlying mechanism responsible for the association, women working night shifts are exposed to light at night which disrupts their circadian rhythm and decreases the release of melatonin.

Melatonin has been shown to be important in maintaining a successful pregnancy, possibly by preserving the function of the placenta.