STUDIES ON PRENATAL MORTALITY IN THE RABBIT, ORYCTOLAGUS CUNICULUS: THE EFFECT OF TRANSFERRING VARYING NUMBERS OF EGGS

Abstract

SUMMARY The relationship between the number of fertilized eggs and prenatal mortality was studied in 160 rabbits by means of egg transfer. More than 3000 fertilized eggs were obtained 60 hr. post coitum from 118 donors, in which superovulation was induced with gonadotrophins. These eggs were transferred to the uteri of recipients which, 72 hr. previously, had been injected with luteinizing hormone. In seventy-one recipients, either 5, 10, 15, 20, 25 or 30+ eggs were transferred unilaterally; in seventy-seven recipients 5 eggs were transferred to one horn and either 5, 10, 15, 20, 25 or 30 + eggs to the other, and in twelve recipients, 10 eggs were transferred to each horn. A further seventeen does were mated with fertile males and used as controls. On day 10, laparotomy was performed in order to count and measure the implantation sites. The numbers of live and dead foetuses and condition of placentae were recorded at autopsy on day 28. Mortality was classified as follows: pre-implantation, days 2½–7; post-implantation, days 7–10 (early deaths); days 10–17 (middle deaths); days 17–24 (late deaths). In recipients the mean number of ovulations was 10·4 (range 5–18). The proportion of recipients in which implantation occurred (pregnancy rate) was 86·9%, compared with 94·1% in controls. Pregnancy rate was not influenced either by number of eggs transferred, or whether the transfer was unilateral or bilateral. With increasing numbers of eggs, the number of implantations also increased, reaching a maximum of twenty-one in one horn, but the proportion of eggs surviving showed a significant decrease. The mean yield of live foetuses showed little variation between groups, there being a marked rise in embryonic mortality as crowding increased. The maximum number of live foetuses observed in one horn was eleven. A total of 124 recipients, containing 1495 implantations, maintained their pregnancy to day 28. Altogether 208 early deaths (13·9%) occurred, there being a slight tendency for the incidence of early deaths to increase with crowding. There were 367 middle deaths, equivalent to 28·5% of the embryos surviving into the middle period. Crowding had a striking effect on the middle death rate, which increased from 16·8% in horns with one to three living embryos to 60·7% in horns with sixteen to nineteen embryos. A further 141 deaths (15·3%) occurred from days 17–24. The incidence of late deaths remained almost constant in horns containing up to seven embryos surviving to day 17, but above this number it was influenced by crowding. Significantly different levels of mortality were observed between the two uterine horns in individual recipients, depending upon the number of eggs transferred. It was concluded that the cause of mortality, associated with crowding and partial loss of litters, was related mainly to local environmental factors, e.g. inadequate uterine secretion (pre-implantation) and lack of placental development, rather than to systemic factors, which more probably accounted for complete loss of litters.