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. Day 15 progesterone was not measured in the Day 7 embryo stage study, and Day 15 progesterone was assumed to be 1.98 times the concentration of progesterone on Day 7 for each cow (based on the relationship between Day 7 and Day 15 progesterone in the Day 15 embryo length study). The vector of model fitted parameters was [a h1 k P0 Pmax] (assuming b = 4.75 d, n = 0.466, and A0 = 1). This analysis provides more accurate estimates of h1 than a due to the weak effect of progesterone on embryo growth to Day 7.