Seasonality in human reproduction: an update.

A seasonal distribution in human natural conception and birth rates has been consistently demonstrated by epidemiological studies in several geographical areas. Possible hypotheses abound to explain this seasonality. Though a seasonal effect on reproduction is well-documented and has been extensively investigated in mammals, information concerning the influence of seasonal variation and its mechanism on human and primate reproduction is scarce. Recent evidence suggests that deterioration in sperm quality during the host summer in sub-equatorial areas, may result in lower conception rates leading to a reduction in the birth rate in spring. In northern countries, however, in regions where a strong seasonal contrast in luminosity exists, activity of the anterior pituitary-ovarian axis and the conception rate are decreased during the dark winter months. In these areas, inversely, a peak in conception rate during summer leading to a maximum in birth rate in spring has been observed. It is believed that seasonality in the ovulation rate may cause this variability. However, changes in the quality of the ovum or in endometrial receptivity which may lead to a greater waste of ovulated eggs and peri-implantation conceptuses at specific times of the year, have also been suggested. These phenomena might have important implications for in-vitro fertilization and gamete intra-Fallopian transfer as well as infertility in general. The seasonal effects, which may influence primate and human fertility and reproduction and its possible mediators, are critically reviewed.