Calibration of the Embryo Growth Model with Our Day 7 Data. The embryo growth model (Equations [1]–[4]) was fitted to data for each year on the effect of circulating progesterone on embryo stage on Day 7. The estimated model parameters are a = 0.0011 ± 0.0021 ng−1∙mL and h1 = 0.36 ± 0.008 log10 cell number∙d−1 in yr 1 and a = −0.008 ± 0.009 ng−1∙mL and h1 = 0.40 ± 0.015 log10 cell number∙day−1 in yr 2 (assuming b = 4.75 d, n = 0.466, and A0 = 1). This very small effect of progesterone on embryo growth up to Day 7 (model parameter a) is consistent with the 5 d required for the embryo to travel from the fallopian tube to the uterus.