The effect of clomiphene citrate on blastocyst development and implantation in the rabbit.

The induction of ovulation with clomiphene citrate (CC) in human patients results in a high ovulation rate but achieves a relatively low pregnancy rate. To clarify the possible role of CC in interfering with the normal reproductive physiology and embryology, we have used our rabbit model and transferred 4-day-old blastocysts from untreated donors to CC-treated pseudopregnant recipients and from CC-treated donors to untreated pseudopregnant recipients to study embryonic development and implantation. Each group was further subdivided into two subgroups, one receiving CC before and the other after ovulation. CC was administered subcutaneously in three consecutive doses of 10 mg/kg body weight. Ovulation was induced with pregnant mare serum gonadotropin (PMS) and human chorionic gonadotropin (hCG). The implantation rate of the control group, evaluated on day 8 of pregnancy, reached 62.0%. When recipients were treated with CC before ovulation, implantation rate was reduced to 18.8% (P less than 0.0002), and to 20.0% (P less than 0.003) when CC was administered after ovulation. The implantation rate of blastocysts transferred from donors, treated before ovulation, is 22.2% (P less than 0.0055), however, reached 70.8% when treatment was started after ovulation. All implantations were analysed microscopically and showed normal morphological features. Our results demonstrate a potential multiple effect of CC, first on the endometrium by altering its receptivity for the implanting conceptus, second, on tubal physiology by altering egg transport, and finally on ovum maturation before ovulation interfering with development of blastocysts. These parameters may all result in rapid decrease in establishment of implantations and in turn in very low pregnancy rates.