Having a physically demanding job or working shifts may lower a woman's ability to conceive, according to a study.
Researchers in the US say previous investigations have shown that some work routines can influence how long it takes to get pregnant and a woman's ability to deliver a healthy baby at full-term.
However, they say no studies have been able to measure physical changes that work could bring about, such as in reproductive hormones and womb function.
To try to answer these questions, a research team led by Harvard TH Chan School of Public Health in Boston assessed the fertility of 473 women attending a fertility clinic. The women, with an average age of 35, were part of a study into environmental and dietary factors that might affect their fertility.
The team measured the number of remaining eggs in a woman's body – known as the 'ovarian reserve' – and levels of a hormone that stimulates the menstrual cycle.
They also looked at 'ovarian response' – the number of mature eggs capable of developing into a healthy embryo in 313 of the women who had completed at least one cycle of in vitro fertilisation (IVF).
The women gave details about their hours of work, their shift patterns and how physically demanding they found their schedule. They were also asked how much time they spent active and inactive during their leisure time.
The researchers found that 4 out of 10 of the women had to move or lift heavy objects around during working hours on a regular basis. Around 22% found their jobs moderately to very physically demanding. Overall, 91% said they worked normal office hours.
The study, published in the journal Occupational and Environmental Medicine, found that women who had physically demanding jobs had a lower reserve of eggs than those who didn't normally lift heavy objects.
Also, the women with physically demanding jobs going through IVF had a lower total reserve of eggs and fewer mature eggs than those whose jobs did not entail heavy lifting.
Furthermore, these differences were even more marked among women working shifts, particularly those rostered on during evenings and nights or where shift patterns fluctuated.
The researchers say this probably reflects disruption to the body's internal clock.
Age and weight also played a part. Women who were overweight, with a body mass index (BMI) of 25 or more, and who had physically demanding jobs, also had fewer mature eggs than those of the same weight who didn't have to do any heavy lifting.
The researchers caution that they can't prove cause and effect because they carried out an observational study using third party data.
Modern Life and Fertility
Commenting on the findings in a statement, Dr Channa Jayasena, clinical senior lecturer in endocrinology at Imperial College London, says: "We already know that physical exertion and stress can inhibit the female reproductive system and make periods less regular or even absent. Workers are increasingly moving away from a traditional 9 to 5 model, and we still know very little about how this could affect our health.
"This study is too small to rule out that the shift and manual labour workers were exposed to something else that made them less fertile. For example, it is possible that they were poorer and therefore had different social conditions or diet, compared with the 9 to 5 workers. In any case, the results help us understand how our work life can impact on our reproductive health."
Alastair Sutcliffe, professor of paediatrics at University College London, comments in a statement: "Human beings like light. When sunlight hits our retinae, the serotonin 'happy hormone' goes up instantly in the brain. Hence we love sunny winter days, but not dank overcast ones. So shift work is not a biologically good way to work and folks who have to do this are known to get many ill health risks such hypertension [high blood pressure]."
He adds: "If trying to optimise fertility, stick to the day job and leave the lifting to their partner."
Occupational factors and markers of ovarian reserve and response among women at a fertility centre,
L Mínguez-Alarcón et al, Occupational and Environmental Medicine
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