Is there a link between CVD and preterm birth?
Night shifts, physical labor, and fecundity
Women who work at night and perform physically demanding jobs may have lower fecundity, according to researchers from Harvard University. Their findings, published in Occupational and Environmental Medicine, are from a first-of-its-kind study that looked at the association between occupational factors and markers of ovarian response in women of reproductive age.
Participants in the prospective cohort were 786 women aged 18 to 45 with mean body mass index (BMI) 23 who planned to use their own gametes and were being treated at an academic fertility center between 2004 and 2015. Unexplained infertility was the primary diagnosis for most of the patients and 80% had undergone examination for infertility but only 48% had previously been treated for the condition.
A questionnaire was used to collect information on the women’s occupational factors and electronic medical records were used to identify reproductive outcomes in the cohort. Compared with women who worked only day shifts, those who worked evening/night/rotating shifts had 2.3 fewer mature oocytes, on average (P<0.001). Moving heavy objects at work was associated with trends toward fewer total oocytes, mature oocytes, and antral follicles (P=0.08, P=0.07, and P=0.06, respectively).
The inverse association between heavy lifting and oocyte yield was stronger in women older than age 37 and with a BMI of at least 25 and when evening/night shift workers were considered as separate category (3.2 fewer mature oocytes in evening/night shift workers versus 8 in women working days). None of the occupational exposures were associated with Day 3 follicle-stimulating hormone or peak estradiol levels. Adjustment for total leisure time physical activity and smoking status did not affect the results.
Their results, the authors said, “suggest that occupational factors may be more specifically affecting oocyte production and quality, rather than accelerating ovarian ageing, in this study population of women attending a fertility centre.” They cautioned that it may not be possible to generalize the findings to couples conceiving without medical intervention and note that they were not able to control for other work factors that might have been correlated with shift and physically demanding work. “Future work in other studies is needed,” the researchers said, “to further disentangle the effects of rotating shift work with and without nights and current versus lifetime exposure to shift work on fecundity.”