A physically demanding job or work schedules outside normal office hours may lower a woman’s ability to conceive, suggests US research.
Heavy lifting at work and shift patterns that involved evenings, nights or rotations of both were associated with poorer egg quality, according to findings published in the journal Occupational & Environmental Medicine.
“These findings have clinical implications”
Previous research has linked occupational factors to fertility, measured in outcomes such as time to pregnancy and the ability to carry a pregnancy to term, said the researchers.
But they claimed no study had previously assessed whether workplace factors might affect a woman’s biological capacity to have a baby.
The researchers looked at indicators of “ovarian reserve” – levels of follicle stimulating hormone, which rise as a woman ages and represent dwindling fertility, and the number of remaining eggs – in 473 women attending a fertility clinic.
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 IVF.
The women, who had an average age of 35, were quizzed about the level of physical exertion required for their job and the hours and patterns worked, as well as leisure time activities.
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 was less demanding.
In addition, compared with those whose jobs did not entail heavy lifting, women going through IVF with physically demanding jobs had a lower total reserve of eggs and fewer mature eggs – reductions of nearly 9% and nearly 14.5%, respectively.
The differences were greater among women working evenings, nights or rotating them, said the researchers from the Harvard TH Chan School of Public Health in Boston, Massachusetts.
Physically demanding shift work can ‘reduce female fertility’
They were found to have fewer mature eggs, on average, than those working shifts within normal working hours.
The differences were even greater among those specifically working evening and night shifts, possibly because of disruption to the body clock, suggested the researchers, led by Dr Lidia Mínguez-Alarcón.
Women who were overweight and whose job was physically demanding also had fewer mature eggs than those of the same weight who did not have to do any heavy lifting at work.
The researchers said: “These findings have clinical implications, as women with fewer mature oocytes would have fewer eggs which are capable of developing into healthy embryos.”
Their findings “suggest that occupational factors may be more specifically affecting oocyte production and quality, rather than accelerating ovarian ageing”, said the study authors.
However, they cautioned that their findings were observational and also may not be applicable to couples attempting to conceive naturally without medical assistance. But they added that they reflected evidence from similar previous studies.
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“Our findings that women who reported moving or lifting heavy objects at work had lower oocyte yields is consistent with a previous study which examined the effect of physical factors at work on menstrual cycle characteristics and fecundity among female nurses in the US.”
In these studies, nurses who reported lifting or moving a heavy load more than 15 times per day had had a 34% higher prevalence of irregular cycles and a 43% longer median duration of pregnancy attempt, compared with women who never lifted heavy loads.
“Taken together with our results, it appears that lower oocyte quality could be one pathway mediating the relationship between high frequency of moving or lifting heavy loads at work and reduced fecundity,” they said.