A physically demanding job, or working shifts 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.
The team of American researchers tested the women’s ovarian reserve – the number of remaining eggs – and their levels of follicle stimulating hormone, which helps control the menstrual cycle and the production of eggs by the ovaries.
Among 313 of the women who had completed at least one cycle of IVF treatment, the researchers also examined the number of mature eggs capable of developing into a healthy embryo.
The women were questioned about their work shift patterns and the level of physical exertion required to do their job.
Nine in ten worked normal office hours. Some 22 per cent said their jobs were moderately to very physically demanding, and 40 per cent reported that they had to lift or move heavy objects at work.
The study authors found that the type of workload did not seem to make any difference to follicle stimulating hormone 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 cautioned that the study is observational, so no firm conclusions can be drawn about cause and effect. And they said as the findings were drawn from a sample of women attending a fertility clinic, it 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 shown to be the main toxin that diminishes ovarian reserve.”