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Women doing heavy or shift work have lower fertility

Poorer egg quality when women’s work is physically demanding or outside ‘office hours’

Lousie Prime

Wednesday, 08 February 2017

Women whose work involves heavy lifting or who work outside normal office hours tend to have poorer egg quality, which might reduce their ability to conceive, according to US research published today in Occupational & Environmental Medicine. The researchers behind the study* said the effect of physically demanding work on a woman’s number of mature eggs was particularly marked if she was also overweight.

In what US researchers said was the first study to look directly at workplace influences on markers of a woman’s biological capacity to have a baby – rather than just on the link between occupational factors and fecundity – they assessed markers of ovarian reserve and response in 473 women, median age 35, enrolled in a prospective cohort study of couples presenting to one academic fertility centre between 2004 and 2015. They measured the number of remaining eggs (antral follicle count) and levels of follicle stimulating hormone (FSH), as well as 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) by December 2015.

They provided information on how often they lifted or moved heavy objects in their current job (‘never’, ‘sometimes’ or ‘often’), the level of physical exertion in that job (light, moderate or heavy) and whether their typical work shift was day, evening, night or rotating; their body mass index (BMI) was also recorded.

Overall, 40% of women reported lifting or moving heavy objects at work, 22% reported moderate or heavy physical exertion at work and 91% worked day-only shifts. The median BMI was 23.2 kg/m2.

The researchers found that women who reported lifting or moving heavy objects at work had 1.0 fewer total oocytes, 1.4 fewer mature oocytes and 0.7 fewer antral follicles compared with women who said they never lifted or moved heavy objects at work. The inverse association between heavy lifting and oocyte yield was stronger in women older than 37 years (compared with younger women) and in those whose BMI was at least 25 kg/m2 (compared with lean women).

Women who worked evening/night/rotating shifts had 2.3 fewer mature oocytes, on average, than women who worked day-only shifts; differences were even greater among those specifically working evening and night shifts, which the researchers suggested might result from disruption of the body clock. None of the occupational exposures were associated with day 3 FSH or peak oestradiol levels.

The study authors said its observational nature meant they couldn’t establish causality; and because it was conducted in a fertility clinic, the findings might not necessarily apply to couples trying to conceive naturally. Nevertheless, they concluded: "Women working non-daytime shifts and those with more physically demanding jobs had fewer mature oocytes retrieved after controlled ovarian hyperstimulation. Our results provide insight into possible mechanisms linking these occupational exposures with decreased fecundity."

* Mínguez-Alarcón L, Souter I, Williams PL et al. Occupational factors and markers of ovarian reserve and response among women at a fertility centre. Occup Environ Med 2017. Published online first doi 10.1136/oemed-2016-103953.

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