CLOCK and spontaneous abortion: A hint that the circadian molecular clock is regulating time-of-day-dependent variation in the reproductive hormones that affect fertility are the recent findings that a polymorphism in the circadian clock-gene ARNTL (rs2278749) confers increased risk of miscarriages, and a polymorphism (rs11673746) in the circadian clock-gene NPAS2 (a CLOCK homolog) may be protective and is associated with decreased incidence of miscarriage (44).