Heavy lifting and shifts may cut fertility

  • The BMJ

Women who do physically demanding jobs or who work shifts may have reduced fertility, research published in Occupational and Environmental Medicine has found.1

The Environment and Reproductive Health (EARTH) study found that heavy lifting at work and non-daytime work schedules were associated with poorer egg quality in women attending an academic fertility clinic.

The researchers examined the ovarian reserve of 473 women, measured by the number of remaining eggs and levels of follicle stimulating hormone. They also looked at the ovarian response—the number of mature eggs capable of developing into a healthy embryo—in 313 of the women, after controlled ovarian hyperstimulation during in vitro fertilisation (IVF).

The women, with an average age of 35 and average body mass index of 23, were asked about their work schedule and level of physically demanding work, as well as other lifestyle factors. In total, 40% of the women reported lifting or moving heavy objects at work, and 22% said that their jobs were moderately to very physically demanding. Nine in 10 of the women worked day-only shifts.

Ultrasound scans showed that the number of remaining oocytes ranged from eight to 17 in the 473 women, while the average number of mature oocytes retrieved from the 313 women undergoing an IVF cycle was nine.

Women who reported lifting or moving heavy objects at work had 1.0 fewer total oocytes (P=0.08), 1.4 fewer mature oocytes (P=0.007), and 0.7 fewer antral follicles (P=0.06) than women who reported never lifting or moving heavy objects at work. The inverse association between heavy lifting and oocyte yield was stronger in women aged over 37 and in overweight or obese women with a body mass index of 25 or higher.

Women who worked evening, night, or rotating shifts had 2.3 fewer mature oocytes on average than women who worked day-only shifts (P<0.001).

The study was observational and had a number of limitations. For example, the researchers asked only about current work patterns and did not assess the impact of other possibly influential factors such as long working hours or switching between day and night shifts.

Alastair Sutcliffe, professor of paediatrics at University College London, commented, “In the study no effort was made to address confounding by testosterone levels in those women. A physically stronger woman is more likely to undertake heavy lifting but would also be implicitly less fertile.”

Channa Jayasena, clinical senior lecturer in endocrinology at Imperial College London, added, “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.”

By Jacqui Wise