According to the "observational" research, heavy lifting and shift work could potentially affect a woman's ability to conceive
A new study has linked shift work and physically demanding jobs to lower fertility among women.
According to the research, heavy lifting and work outside normal office hours could have a potential effect on a woman's ability to conceive.
The US study, published in the Occupational and Environmental Medicine journal, examined 473 women attending one fertility clinic.
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 that were able to develop into a healthy embryo.
The women were also asked about their work shift patterns and how much physical exertion was required to do their job.
The researchers found that women with physically demanding jobs had a lower reserve of eggs than those whose work did not require regular heavy lifting.
Among the studied women, those with physically demanding jobs had a lower total reserve of eggs and fewer mature eggs.
The type of workload was not seen to make any difference to FSH levels.
Women working evening, night or rotating shift patterns were found to have fewer mature eggs than those working normal hours.
The authors warned that the study is observational - so no firm conclusions can be drawn about cause and effect.
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,” he said.
"It is important to note that there was no difference in smoking status between the groups.”
Nine in 10 of the women observed said they worked normal office hours while 22% said their jobs were moderately to very physically demanding.
Some 40% of women reported lifting or moving heavy objects at work.
The study focused solely on women attending a fertility clinic and the findings may not apply to those trying to conceive naturally.