The Distribution in Embryo Size on Days 7 and 15. The embryo growth model (Equations [1]–[5]) was used to predict embryo size on Day 7 from the measurements of progesterone (Days 0, 7, and 15) and embryo length on Day 15 (yr 1: farms 1–3; yr 2: farms 1 and 4). The predicted distributions in embryo size on Day 7 is shown in Figure 1C (yr 1: farms 1–3; yr 2: farms 1 and 4), which have a similar shape to the measured distribution in embryo size on Day 7 in Figure 1A. The similar variance in measured and predicted (based on Day 15 length) Day 7 embryo stage indicate very low stochastic perturbations in embryo growth in vivo. Also, no evidence supports an effect of embryo size on Day 7 on embryo survival to Day 15. This is consistent with our previous ET trial where no effect was observed of transferred embryo size on embryo survival from Day 7 to 15 (Shorten et al., 2018).