Implantation and Later Fetal Development in Immature Rats Given a Superovulatory Dose of Pregnant Mare′s Serum Gonadotropin, Later Neutralized by Antiserum1

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Abstract
In an earlier experiment, 29-day-old female rats were superovulated with 40 IU pregnant mares 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. Later development of these blastocysts were investigated. Implantation occurred in only .apprx. 50% of superovulated (SOV) a/s-treated animals regardless of whether or not estrogen was given on day 4. 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. In a significant percentage of SOV a/s animals blastocysts are capable of implanting and continuing to develop into normal fetuses by day 20. At least 50% of SOV a/s-treated rats fail to maintain pregnancy between days 5 and 8; this could be a result of a failure in the uterus to provide a suitable environment for implantation or to abnormalities in the blastocysts.