Dose-response effects of equine chorionic gonadotrophin (eCG) and human chorionic gonadotrophin (hCG) on early embryonic development and viable pregnancy rate in rats

The present study examined the dose-response effects of eCG treatment alone and in combination with various doses of hCG on early embryonic developmen t in vivo and viable pregnancy rate in rats. Mated female Wistar rats were treated with eCG alone (0, 10, 20 or 40 iu), or with 20 iu eCG in combinati on with various doses of hCG (10, 20, 40 or 80 iu) administered 48 h later. The animals were killed on days 2, 3, 4, 5 or 14 of pregnancy and the numb ers of embryos and fetuses recovered were scored. All rats treated with 0 o r 10 iu eCG were pregnant. The pregnancy rate was reduced from 62.5% on day 2 to 25% on day 14 and from 31% on day 2 to 10% on day 14 in the groups tr eated with 20 and 40 iu eCG, respectively. The reduction in pregnancy rate induced by 20 iu eCG was negated by the increasing doses of hCG used. A 100 % pregnancy rate was noted on days 2 and 3 in the groups treated with doses of hCG between 10 and 80 iu and from day 2 to day 4 in the groups treated with doses of hCG between 20 and 80 iu. However, a higher viable pregnancy rate was observed only in the group treated with 10 iu hCG compared with th e group treated with 20 iu eCG and 0 iu hCG. These results imply that hyper stimulation of rats with high doses of eCG compromises pregnancy rate and m arkedly reduces litter size and that the addition of hCG is required for co mplete ovulation, which results in higher embryo yield and a delay in early embryo demise.