The Influence of Intrauterine Factors on the Fetal Weight of Rabbits

Observations were made on 475 fetuses carried by 71 pregnant rabbits. 63 or 88.7 per cent of the 71 does were sacrificed from 28 to 31 days after the last fertile mating, and these does bore 401 or 88.2 per cent of the total of 455 fully developed fetuses. The following information was available with reference to each fetus: age, weight, weight of corresponding placenta, horn, i.e., right or left, presentation, and position or order. The presentation indicated that part, head or breech, which was directed toward the vagina, and position or order, the relative locus of the fetus in the horn, the first position being that nearest the ovary. As the gestation period approached its normal limit of 31 days, the relative daily increase in mean fetal weight was progressively retarded. There was no significant difference between the number of fetuses in each uterine horn. Head presentation was significantly more frequent than breech, but the uterine horn in which the fetus was located had no influence on its presentation. A greater relative number of breech presenting fetuses was observed in the third position than in the other positions. Presentation did not exert a significant influence on fetal weight. Fetal weight at or near term was significantly influenced by the position or order in the uterine horn. In general, the weights of fetuses implanted high up nearest the ovary were greater than those developing nearest the outlet, and fetuses occupying intermediate positions had intermediate weights. When, however, only two fetuses were present in a horn, position had no effect on their weights. A significant positive coefficient of correlation was observed between fetal and placental weights. Moreover, placental weight was influenced by position in the uterine horn in exactly the same manner that fetal weight was so influenced. The factors which produced variability in fetal weight at or near term, did not account for the abnormally low birth weights of the dwarf rabbits observed in this laboratory.