Ovulation occurs on 1 day during each menstrual cycle (even if multiple follicles are involved), and the several days preceding ovulation are when intercourse is most likely to result in pregnancy. Collectively, the potentially fertile days up to and including the day of ovulation are called the “fertile window” (1). Since the 1930s, several biomarkers of the fertile window have been investigated for the purposes of empowering women and couples seeking to use fertility-awareness based methods or natural family planning to avoid pregnancy, as well as to expand understanding of human reproductive physiology.