Abstract
The relation between the number of implanted embryos and available uterine space per fetus in intact does, and also the effect of available uterine space on the development of the fetus and the survival rate at 18 days of gestation were examined. Rabbits came from the sixth generation of a divergent selection experiment on uterine capacity. The high and low lines showed the same ovulation rates, but the high line had a higher rate of implantation and number of live fetuses ( P < 0.05). As a consequence of the higher degree of uterine overcrowding in the high line, the length of full uterine horn was 10% higher ( P < 0.10) and the available uterine space per live fetus was 20% lower ( P < 0.05) than in the low line. The available uterine space per implantation site showed a negative quadratic regression coefficient with the number of implanted embryos ( P < 0.001) and a negative linear regression coefficient with the number of dead fetuses (− 0.19 ± 0.10). The available uterine space had a quadratic relation with the length of maternal placenta ( P < 0.001), and it was linearly associated to development of fetal placenta and fetus ( P < 0.05). The fetal position within the uterus did not affect the proportion of dead embryos. However, the fetuses with placentae receiving fewer than three blood vessels showed a higher probability of death ( P < 0.01) and a smaller uterine space ( P < 0.05) than those receiving more than three blood vessels. A poor vascular supply and reduced uterine space could affect the subsequent fetal development.
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