where g is a measure of the rate of embryo elongation (log10 mm·d−1), n describes the nonlinear relationship between uterine competency and embryo elongation, AH (log10 cell number) is the embryo stage when elongation is initiated and tH (d) is the hatching time. The number of cells and length of a hatching blastocyst have been measured (Berg et al., 2010; Spencer, 2013), and we assume that LH = log10(0.3) log10 mm and AH = log10(512) log10 cells.