The progesterone content of corpora lutea from armadillos in the delayed implantation period and both corpora and placentae from animals in the post-implantation period was determined chemically.
During the delay period the weight of the corpus luteum, but not the total progesterone content (μg/gland), was significantly higher in animals carrying blastocysts than animals without blastocysts. Following implantation of the blastocyst, the size as well as the total progesterone content increased significantly (P<0·05). However, during the latter part of the post-implantation period (> 6 cm crown-rump length) both the size and the progesterone content decreased by more than two-fold (P<0·05). However, at this stage considerable amounts of progesterone were found in placentae and in one uterine vein blood sample studied. In contrast, no hormone could be detected in the placenta of earlier stages (i.e. <5 cm crown-rump length).
These data suggest that the functional activity of the corpus luteum may be lower during delayed implantation than in the early post-implantation period. However, during the latter part of gestation the corpus luteum regresses, and it is probable that at this stage the placenta contributes significant amounts of progesterone.