Pregnant women who work night shifts are nearly a third more likely to suffer a miscarriage, a study has found.
Working two or more night shifts the previous week raised the risk by 32 per cent, according to a study of 20,000 women.
And Danish researchers now warn the more night shifts worked, the higher the chance of losing a baby.
Scientists fear too much light exposure during the night disrupts the body's internal clock and cause levels of melatonin to drop.
Pregnant women who work night shifts are nearly a third more likely to suffer a miscarriage, scientists have warned
Melatonin is a hormone that preserves the function of the placenta and is crucial in securing a successful pregnancy.
The study adds to previous findings, but is the first to quantify the amount of night shifts involved in the increased miscarriage risk.
The researchers at the Bispebjerg and Frederiksberg Hospital in Copenhagen said it could have implications for the hours women work during pregnancy.
Scientists led by Dr Luise Moelenberg Begtrup analysed the working lives of more than 22,744 expecting women from five different regions over six years.
The study focused on women working in the public sector, including hospitals.
Almost half of the women worked nine night shifts throughout their pregnancy and 2,000 - eight per cent - of the women suffered a miscarriage.
A miscarriage is the loss of a pregnancy during the first 23 weeks, according to the NHS, but this study only studied miscarriages up until week 22.
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.
Researchers are yet to determine a direct cause, and point out that the data on miscarriages, particularly early miscarriages, were incomplete.
RISK OF A MISCARRIAGE SPIKES TO 54% FOR OVER-45s
The risk of suffering a miscarriage soars for older mothers, a study suggests.
It rises slightly to 11 per cent among women in their early 30s and to 17 per cent among women aged 35 to 39.
The risk of miscarriage then spikes to 32 per cent among those in their early 40s and 54 per cent over the age of 45.
The research, by the Norwegian Institute of Public Health, involved 420,000 women who became pregant between 2009 and 2013.
They also found a strong recurrent risk of miscarriage - with a woman who had already suffered one miscarriage having a 50 per cent increased risk of a repeat.
After two, the risk doubled, and after three consecutive miscarriages, the risk was four times greater.
Previous pregnancy complications also predicted a higher risk of miscarriage.
The number of older mothers has increased in recent decades, as more women concentrate on their career first and turn to family later.
But they suspect women being exposed to light during the night time could be an important contributing factor.
Light exposure during the night can disrupt a woman's circadian rhythm which decreases the release of melatonin.
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.'
Previous studies have suggested pregnant women face a greater risk of miscarriage if they work night shifts, but they have been based on self-reported shift work.
For this study, published in Occupational & Environmental Medicine, the authors accessed payroll data and linked that with data from Danish national registers on births and admissions to hospital for miscarriage.
Dr Begtrup said the results are of importance because 14 per cent of pregnant women work at least one night shift a month.
She said: '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.'