Follicular development and induction of ovulation with human chorionic gonadotropin in the pregnant rat

Aims: The number of ovarian follicles and ovulations after human chorionic gonadotrophin (hCG) was given at various doses during pregnancy were investigated in rats. Methods: On designated days of pregnancy, rats were injected with hCG at 13.00 hours and the number of ova were observed. The same rats were then killed at 13.00 hours the next day. The ovaries were cut into complete sections. All sections were observed and the size of each follicle was measured. Results: The number of large (≥550 µm), healthy, non-atretic follicles (Graafian follicles) ranged from 1.5 to 11 during pregnancy. Ovulation was induced in rats at any stage of pregnancy with intramuscular injections of 40 or 50 IU hCG, but 10 IU had no effect and 20 or 30 IU had an incomplete effect. The number of ovulations after 40 IU hCG was 2-12 and there was not a significant difference between the number of ovulations and the number of large, healthy follicles at any point in the pregnancy. Conclusion: Graafian follicles existed through the pregnancy. Results suggest that although hCG induces ovulation in large ovarian follicles (≥550 µm) during pregnancy, the responsiveness to hCG is lower than in recurrent estrus-stage rats. (Reprod Med Biol 2004; 3: 201-204).