where t is time from ovulation (days), k is the rate of progesterone production (ng−1∙mL∙d−1), Pmax is the maximum progesterone concentration (ng∙mL−1), and P0 is the Day 0 progesterone concentration (ng∙mL−1; Shorten et al., 2010). This model describes the progesterone dynamics during the initial stage of the cycle (Tsai and Wiltbank, 1998).