Working shifts may affect women's fertility, a new study indicates.

According to UK researchers, shift work often leads to sleep deprivation and disruptions to the circadian rhythm (body clock) and this has been previously linked to an increased risk of certain conditions, such as breast cancer.

However the effect of working shifts on reproductive health is largely unknown.

The researchers from the University of Southampton in the UK carried out a detailed analysis of all studies on this topic that were published between 1969 and the start of 2013. These looked at non-standard working schedules, such as night shifts, and standard working schedules and included almost 120,000 women.

The study found that women who worked irregular hours - night shifts, evening shifts and alternating shifts - had a 33% increased risk of experiencing disruption to their menstrual cycle compared to women who worked regular hours.

They also had an 80% increased risk of experiencing subfertility (reduced fertility).

The study also found that women who worked night shifts only did not have an increased risk of menstrual disruption and subfertility. However, they did have an increased risk of suffering a miscarriage. This pattern was not seen in women who worked night shifts mixed with other types of shift.

The researchers pointed out that while they have found a link between shift work and ‘negative early reproductive outcomes', the actual cause has not been proven.

"However, if our results are confirmed by other studies, there may be implications for shift workers and their reproductive plans. More friendly shift patterns with less impact on circadian rhythm could be adopted where practical - although the optimal shift pattern required to maximise reproductive potential is yet to be established," said Dr Linden Stocker.

Acknowledging that only certain areas of reproduction were affected, she emphasised that issues relating to reproductive problems are ‘complex'.

"Indeed, it is probable that completely different causes underlie menstrual dysfunction, miscarriage and subfertility. This may explain why the effects of different types of shift work are seen in some groups of women, but not others," she added.

Details of these findings were presented at the annual conference of the European Society of Human Reproduction and Embryology (ESHRE) in London.