STUDIES ON PRENATAL MORTALITY IN THE RABBIT, ORYCTOLAGUS CUNICULUS: THE AMOUNT AND DISTRIBUTION OF LOSS BEFORE AND AFTER IMPLANTATION

Abstract

SUMMARY The amount and distribution of prenatal mortality were studied in 126 first and second pregnancies in seventy-three rabbits, weighing 3·5–4 kg. The does were mated naturally, artificially inseminated to ensure against the failure of natural mating, and injected with 25 i.u. luteinizing hormone. The numbers of corpora lutea and implantation sites were counted at laparotomy 7–13 days post coitum (p.c.) and the number of live foetuses was recorded at autopsy 28 days p.c., or after parturition. The diameter of the uterine swellings was measured at laparotomy. In a further eight pregnancies only the litter size was recorded. Ovulation occurred in every case, but in six (4·8%) of the 126 pregnancies all the ova (109, or 8·1%) were lost before implantation. In the remaining 120 pregnancies there was a total loss of 132 ova (or 154 if corrected for polyovuly), representing 9·7% of ovulations. In 64 out of the 120 pregnancies (53·3%) there was some loss of ova before implantation. The proportion of litters showing loss and the proportion of ova lost before implantation were significantly related to the number of ovulations. After implantation, total resorption occurred in four pregnancies (3·3%) and in a further eighty-five pregnancies (70·8%) there was some loss of embryos, which altogether amounted to 18·3% of the ovulations. The proportion of litters suffering loss after implantation was significantly related to the number of implantations. It was concluded that 7% of the embryos died shortly after implantation, 66% died between days 8 and 17, and 27% were lost between days 17 and 23. Data relating to the diameter of 1728 uterine swellings measured between days 7 and 20 p.c., counts of corpora lutea, the occurrence of corpora lutea atretica and polyovuly, the position of foetuses in utero, weights of placentae associated with resorbed embryos, foetal weights, birth weights, and the incidence of stillbirths are also presented.