CONCLUSIONSProgesterone is not a strong predictor of in vivo embryo growth to Day 16. In fact, in vivo embryos are 3 times less sensitive to progesterone than in vitro-transferred embryos (up to Day 15). These results highlight a need for caution when extrapolating the results of in vitro ET studies to the in vivo situation. The similar variance in measured and predicted (based on Day 15 length) Day 7 embryo stage indicate low stochastic perturbations for in vivo embryo growth. Furthermore, Day 7 embryo stage could be well predicted based on the gene expression profile.