Shift work and physically demanding jobs have been linked to lowered fertility among women, a new study suggests.
A physically demanding job or working outside of normal office hours may lower a woman's ability to conceive, the researchers found.
Their study, published in the journal Occupational and Environmental Medicine, examined 473 women attending one fertility clinic.
A team of US researchers tested the women's ovarian reserve - the number of remaining eggs - and their levels of follicle stimulating hormone (FSH).
Among 313 of the women who had completed at least one cycle of IVF, the researchers also examined the number of mature eggs capable of developing into a healthy embryo.
Meanwhile, the women were also quizzed about their work shift patterns and the level of physical exertion required to do their job.
Nine in 10 worked normal office hours and 22% said their jobs were moderately to very physically demanding and 40% of women reported lifting or moving heavy objects at work.
The authors found that type of workload did not seem to make any difference to FSH levels.
But women with physically demanding jobs had a lower reserve of eggs than those whose work did not regularly require heavy lifting.
And among women going through IVF, those with physically demanding jobs had a lower total reserve of eggs and fewer mature eggs, the researchers found.
The differences were even greater among women working either evening, night or rotating shift patterns.
These women had fewer mature eggs than those working normal working hours.
The authors caution that the study is observational, so no firm conclusions can be drawn about cause and effect. And they said the findings were drawn from a sample of women attending a fertility clinic so may not apply to those trying to conceive naturally.
Professor Adam Balen, chairman of the British Fertility Society, said: "In this interesting study, an association has been shown between physically demanding jobs and lower potential fertility.
"It is difficult to hypothesise a mechanism by which a physically demanding job may have a negative effect on ovarian reserve, as the number of eggs (oocytes) is determined at birth and lost progressively throughout life, with smoking having been shown to be the main toxin that significantly diminishes ovarian reserve.
"It is important to note that there was no difference in smoking status between the groups.
"I wonder therefore if there may have been maternal influences on the women studied that could have effected their ovarian reserve at birth, for example maternal smoking and nutrition, which might then have some bearing on the future reproductive health of their daughters."