Shift work, heavy lifting may reduce women's fertility, research shows

Women who work nights and do heavy lifting on the job may have fewer and inferior eggs than nine-to-fivers, said a study into a potential cause of infertility.

Previous research had shown a link between work conditions and fertility, but this was the first attempt to examine how shift work or manual labour might affect a woman's physical capacity to have a baby, the authors said.

The U.S.-based team analyzed data from about 400 women attending a Massachusetts fertility clinic. Their average age was 35.

The researchers assessed "ovarian reserve" -- the number of remaining eggs a woman had -- as well as levels of a hormone which rise as fertility dwindles.

They also examined how many eggs were "mature" and capable of developing into a healthy embryo.

This data was compared to the women's job conditions -- how much physical exertion was required of them, and the hours that they worked.

About 40 per cent of women said they had to regularly move or lift heavy objects, while a fifth said their jobs were moderately to very physically demanding.

Ninety-one per cent worked during normal office hours.

"Women with physically demanding jobs had a lower reserve of eggs than those whose work did not regularly require heavy lifting," said a press statement from the journal Occupational & Environmental Medicine, which published the study.

Heavy lifters also had fewer mature eggs -- even fewer if they worked evening, night or rotation shifts.

The effect was strongest in overweight women and those older than 37, the researchers said.

Women are believed to be born with a finite number of eggs, which steadily die off over time -- a process which can be accelerated by factors such as smoking, for example.

The team merely observed a correlation between work conditions and egg health, and cannot say with certainty that one causes the other.

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