Implantation and later fetal development in immature rats given a superovulatory dose of pregnant mare's serum gonadotropin, later neutralized by antiserum.

In an earlier experiment, 29-day-old female rats were superovulated with 40 IU pregnant mare's serum gonadotropin (PMSG) and very few blastocysts were recovered from the uterus on Day 5. Administration of a PMSG antiserum (a/s) prior to ovulation resulted in recovery of blastocysts in all rats and the present set of experiments was undertaken to investigate the later development of these blastocysts. Implantation was found to occur in only approximately 50% of superovulated (SOV) a/s-treated animals regardless of whether or not estrogen was given on Day 4. It is suggested that the exposure to high preovulatory estrogen and an imbalance in the progesterone/estrogen ratio during the early preimplantation period are possible causes of the loss of blastocysts. The majority of SOV a/s-treated rats in which implantation occurred carried fetuses to Day 20, although there was a further small loss of pregnancy between Days 8 and 20. On Day 8 steroid concentrations in both serum and ovaries were similar to those observed on Day 5 in earlier experiments. By Day 20 serum and ovarian progesterone concentrations were significantly higher than on Day 8 in all superovulated rats, which may reflect increased production by the larger number of corpora lutea in these animals. The present set of experiments shows that in a significant percentage of SOV a/s animals blastocysts are capable of implanting and continuing to develop into normal fetuses by Day 20. However, at least 50% of SOV a/s-treated rats fail to maintain pregnancy between Days 5 and 8 and this could be a result of a failure in the uterus to provide a suitable environment for implantation or to abnormalities in the blastocysts.