The overall average embryo length was 53.5 ± 3.7 mm in yr 1 and 52.8 ± 4.5 mm in yr 2 when embryos recorded as a minimum length were set to the recorded minimum length (a total of 155 embryos in yr 1 and a total of 110 embryos in yr 2). Embryo length is approximately log normally distributed in each year (Figure 1B), consistent with the exponential growth process. Log embryo length is not significantly different between farms in yr 1 (Supplemental Figure S2; https:// doi .org/ 10 .3168/ jds .2017 -14306) and yr 2 (Supplemental Figure S3; https:// doi .org/ 10 .3168/ jds .2017 -14306). In yr 1 no significant effect was observed of Day 7 progesterone on Day 15 embryo length for farm 4, and the effect of Day 7 progesterone on Day 15 embryo length was significantly different between farm 4 and farms 1 to 3. The effect of circulating Day 7 progesterone on embryo length on Day 15 (embryos recorded as a minimum length were set to the recorded minimum length) is shown in Figure 2B for yr 1 (R2 = 0.14, P < 0.001 for farms 1 to 3; n = 87 embryos) and yr 2 (R2 = 0.04, P = 0.04 for farms 1 and 4; n = 109 embryos). Similar relationships are observed if embryos recorded as a minimum length were excluded from analysis (yr 1: R2 = 0.14, P < 0.001 for farms 1 to 3; yr 2: R2 = 0.04, P = 0.05 for farms 1 and 4).