The low implantation rate after embryo transfer (ET) is the most important problem to be solved in in vitro fertilization and embryo transfer (IVF-ET). Various factors which may affect embryo implantation in the endometrium have been examined. In this study, the influence of the timing of luteinization on the establishment of pregnancy was investigated. Follicle stimulation was performed by luteinizing hormone-releasing hormone agonist, human menopausal gonadotropin and human chorionic gonadotropin (hCG). hCG was injected on the day when the mean diameter of two follicles exceeded 16 mm, and 36 h after the injection the oocyte was retrieved. Progesterone (P4) supplement was started on the day of ovum retrieval (50 mg), followed by 30 mg for 14 days. The patients were divided into three groups: (1) the pregnant patients (n = 20: group Pre); (2) the nonpregnant patients without bleeding during P4 injection (n = 35; group N), and (3) the nonpregnant patients with bleeding during P4 injection (n = 30; group B). The number of follicles that developed, endometrial thickness, the day of operation, the number of oocytes harvested, the number of good quality oocytes, the number of oocytes fertilized, the number of embryos transferred, estradiol (E2) and P4 levels, and the E2/P4 ratio were examined. The endometrial thickness of group Pre (10.9 ± 0.6 mm; mean ± SEM) on the day of hCG injection was significantly (p < 0.05) greater than that of groups N (9.0 + 0.4 mm) and B (9.2 ± 0.3 mm). The number of transferred embryos in group Pre (2.8 ± 0.3) was also greater (p < 0.05) than in groups N (1.4 ± 0.2) and B (1.7 ± 0.2). The mean level of E2 did not show any difference among the three groups, but the P4 level of group B (18.2 ± 1.2 ng/ml) on the day of ET showed significantly (p < 0.01) lower values when compared with group Pre (30.7 ± 4.2 ng/ ml). The E2/P4 ratio on the day of both ovum pick-up and ET was significantly higher in group B than in groups Pre and N, suggesting a delay of luteinization in group B, which might have induced the breakthrough uterine bleeding, with the resultant failure to establish a pregnancy. The E2/P4 ratio of group Pre (2,024 ± 488) was higher (p < 0.05) than that of group N (1,029 ± 180) on the day of hCG injection. This implies that more follicles were developed in group Pre than in group N. Thus the establishment of pregnancies in IVF-ET was produced by the thicker endometrium, the larger number of embryos transferred, and the more rapid luteinization of follicles after hCG injection.

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