B, Serum P4 levels on days 4 and 6 of pregnancy were comparable between Rb1f/f and Rb1d/d mice. In Rb1d/d mice, pre-implantation P4 treatment on days 2 and 3 increased serum P4 levels on day 4 but did not affect those on day 6 (mean ± SEM, Student's t test; n=4 mice for each group). 