Women who have physically demanding jobs or non-daytime working hours may experience decreased fertility, according to a study.
Previous studies had suggested a possible link between non-standard working hours or physically taxing jobs and reduced fertility, but this is the first to try and link such work factors to biological markers. The results appear to be strongest in older, overweight or obese women.
'Our study suggests that women who are planning pregnancy should be cognisant of the potential negative impacts that non-day shift and heavy lifting could have on their reproductive health,' said Dr Lidia Mínguez-Alarcón of the Harvard TH Chan School of Public Health, lead author on the study, published in Occupational and Environmental Medicine.
The small study, carried out between 2004 and 2015, measured biological markers of fertility – including hormone levels and the number of immature eggs in the ovaries – in 473 women who were already seeking treatment for infertility.
Of these, 313 went on to have at least one cycle of IVF treatment. Researchers recorded the total number of eggs retrieved and the number of mature eggs with the potential to develop into healthy embryos.
They compared the outcome of these tests with the physical demands of jobs and working schedules of the women, as reported by the test participants on a questionnaire.
Women who reported lifting heavy objects at work had 8.8 percent fewer total eggs and 14.1 percent fewer mature eggs compared with women working in less manual jobs. The effect was strongest in overweight or obese women and those aged 37 or older.
Similarly, working nights or rotating shifts also appeared to negatively affect the number of mature eggs.
However, as all of the test participants were already seeking treatment for infertility, these findings may not be a true reflection of factors affecting fertility in the wider population.
Furthermore, only 313 women underwent IVF and could provide data about egg retrieval. Of these, 106 reported physically demanding work, and 36 worked night or rotating shifts. The study did not assess any other lifestyle or health factors of the women involved.
'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,' said Dr Channa Jayasena, clinical senior lecturer in endocrinology at Imperial College London, who was not involved with the study.