The influence of occupational physical activity on the menstrual cycle and fecundability.

We examined the influence of the level of occupational physical activity per working hour (intensity score) and per working day (fatigue score) on menstrual function and fecundability in a population of 260 nonmedical female workers who were employed at 39 Dutch hospitals and were planning a pregnancy. We studied the influence of the intensity and fatigue scores as such and in combination with unfavorable working hours (before 8:00 am or after 6:00 pm) and high working speed (working at high speed for more than 50% of the time). The data did not reveal a clear relation between the occupational physical activity levels (representing an estimated energy requirement of up to 3 times the basal metabolic rate) and menstrual function, but the presence of a small effect could have been missed. The fecundability of women with a high fatigue score, particularly in association with unfavorable working hours, was lower than that of women with a low fatigue score (fecundability odds ratio = 0.37; 95% confidence interval = 0.18-0.77). Moderate levels of physical activity, as found in cleaners, kitchen staff, and clerical workers at hospitals, appear to affect the female reproductive system. Fecundability, as measured by the time to pregnancy, seems to be a more sensitive parameter for these biological changes than menstrual function.