Pregnant women who work night shifts are nearly a third more likely to suffer a miscarriage, scientists have warned.
And it might be down to a lack of light, leading to low levels of melotonin which disrupts the body clock.
Danish researchers who analysed the working lives of more than 20,000 expecting women said the dangers of miscarrying dramatically increase the more night shifts pregnant women work.
According to the study, women who work two or more nights a week are 30 per cent more likely to suffer a miscarriage the following week and the risk is becomes greater after week eight.
Almost 2,000 of the women who took part in the survey suffered a miscarriage during the six year period.
Doctor Luise Moelenberg Begtrup, of the Bispebjerg and Frederiksberg Hospital in Cophenhagen, said the groundbreaking results is of national importance because 14 per cent of pregnant women work at least one night shift a month.
She said: "Observational studies indicate an association between working nights and miscarriage, but inaccurate exposure assessment precludes causal inference.
"Using payroll data with exact and prospective measurement of night work, the objective was to investigate whether working night shifts during pregnancy increases the risk of miscarriage.
"This may be explained by the decline in the proportion of chromosomally abnormal foetuses with gestational age.
"This makes an association with environmental exposure more easily detectable among later miscarriages.
"The results could have implications for national occupational health regulations."
The researchers observed the working lives of 22,744 women from five different regions across a six year period.
Almost half of the women worked nine night shifts throughout their pregnancy and eight per cent of the thousands surveyed suffered a miscarriage.
It is the first to analyse women's payroll data and assess how risk of miscarriage increases over time.
Scientists linked payroll data to national registers on birth and hospital admissions for miscarriage to determine how the risk of miscarriage between weeks four to 22 of pregnancy was influenced by night work.
After week eight of pregnancy, pregnant women who worked two or more night shifts the previous week had a 32 per cent higher risk of miscarriage compared to women who did not work any night shifts that week.
It was also discovered that the risk of miscarriage increased the more night shifts worked and how regular women worked nights.
Researchers are yet to determine a direct cause of the link but they suspect women being exposed to light during the night time could be a contributing factor.
Light exposure during the night can disrupt a woman's circadian rhythm which decreases the release of melatonin - a hormone that preserves the function of the placenta and is crucial in securing a successful pregnancy.
Dr Moelenberg Begtrup said: "Studies in humans have found lower levels of melatonin mediated by exposure to light-at-night and with no full catch-up during the day among night workers.
"Melatonin is primarily synthesised in the pineal gland but also in peripheral organs such as the placenta and ovaries.
"It plays a role in preserving the optimal function of the placenta and experimental studies have demonstrated the importance of tightly regulated circadian rhythms, in which melatonin also has a pivotal role, in the maintenance of successful pregnancies.
"The study corroborates earlier findings that night work during pregnancy may confer an increased risk of miscarriage and it indicates a lowest observed threshold level of two night shifts per week.
"The new knowledge has relevance for working pregnant women as well as their employers, physicians and midwifes.
"Moreover, the results could have implications for national occupational health regulations."