February 7, 2017

expert reaction to study looking at role of non-daytime shift work and physically demanding jobs on women’s fertility

Publishing in Occupational & Environmental Medicine new research claims women working non-daytime shifts and those with physically demanding jobs had fewer mature oocytes – on average – compared with women who worked day-only shifts.

A Before the Headlines analysis accompanied these comments.

Prof. Darren Griffin, Professor of Genetics, University of Kent, said:

“The key message of this paper is that women who undertake heavy lifting and/or work unsociable shift hours produce significantly less mature eggs.  It seems that the effect is more exaggerated with increased age and body mass index. The authors measured a number of occupational factors but these were the only ones that showed marked results. The association with these factors is, in itself, nothing new however it is the means by which the production of mature eggs was measured (through an IVF setting) that make the results more convincing.

“The authors also claim to have provided insight into the mechanisms linking these occupational exposures with decreased fecundity. This is perhaps a little over-stated since the study is an associative one with very little mention of mechanisms. That said, women who are trying to start a family may wish to take the study into account, perhaps avoiding heavy lifting and unsociable work hours as much as is possible during this time, especially if they are not falling pregnant within the first year of trying.”

Prof. Adam Balen, Chair of the British Fertility Society said:

“In this interesting study, an association has been shown between physically demanding jobs and lower potential fertility. It is difficult to hypothesise a mechanism by which a physically demanding job may have a negative effect on ovarian reserve, as the number of eggs (oocytes) is determined at birth and lost progressively throughout life, with smoking having been shown to be the main toxin that significantly diminishes ovarian reserve.

“It is important to note that there was no difference in smoking status between the groups. I wonder therefore if there may have been maternal influences on the women studied that could have effected their ovarian reserve at birth (e.g. maternal smoking & nutrition), which might then have some bearing on the future reproductive health of their daughters (that is of the study population).”

Dr Channa Jayasena, Clinical Senior Lecturer in Endocrinology, Imperial College London and Member of the Society for Endocrinology, said:

“The authors observed that in nearly 500 women, doing heavy lifting and/or shift work outside of office hours, there was a decreased response to IVF fertility treatment. 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.”

Prof. Alastair Sutcliffe, Professor of Paediatrics, UCL said:

“Consistent with previous research in this field this American study, a report from the ongoing study aptly entitled ‘ Environment and Reproductive Health (EARTH) Study, suggested that women undergoing fertility treatment are less likely to get a good egg yield if they a) lift heavy objects or b) work shifts.

“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. The typical and consistent differential between the sexes of strength is 10-15% and this accounted for by testosterone. Women also produce testosterone albeit at a lower level than men.

“The second finding of this ‘EARTH study’ is that shift workers are also at risk of reduced egg yield. 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.

“So what does this study mean? 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’ by Mínguez-Alarcón et al. will be published in Occupational & Environmental Medicine on Tuesday 7th February 2017.

Declared interests

Prof. Adam Balen: ‘No COIs’

Dr Channa Jayasena: None received

Prof. Alastair Sutcliffe: None received