Inbred (C57BL♀ x C57BL♂) and F1 hybrid (C57BL♀x C3H♂) litters were compared with respect to (a) birth weight, (b) foetal weight at 17½ days' gestation, (c) placental weight at the same stage. F1 hybrids were heavier than inbred young at birth, but not to any significant extent at 17½ days. Placentae in F1 litters were nearly 20% heavier than those in inbred litters.
Other factors examined were the number of young in the litter and in the uterine horn, and the position of implantation within the uterine horn. Birth weight and foetal and placental weight at 17½ days' gestation were all inversely related to the number of young in the litter. The effect on placental growth was systemic, i.e. it was not affected by the distribution of implantations between uterine horns. There was a suggestion of a systemic effect on foetal growth, in addition to a significant local effect exerted by the other implantations in the same horn. When allowance was made for the number in the litter, those litters distinguished by higher mean placental weight did not necessarily have large foetuses. But within a female, the horn containing the heavier placentae on average had heavier foetuses too, allowance again being made for the number of implantations. Within the uterine horn also, foetal and placental weight showed a positive association. Since there was no genetic heterogeneity among litter-mates, this association suggests a causal dependence of foetal growth on placental size.
The mean weight of foetuses and placentae decreased from the ovarian towards the cervical end of the uterine horn. This effect was most marked for placentae. Foetal growth was not affected directly by position in the uterine horn, but reflected the gradient in placental growth.
It is argued that haemodynamic factors are responsible (i) for the effect of position in the horn on placental growth, and hence on foetal growth, and (ii) for the systemic effect of number in the litter on foetal and placental growth. But the local effect of other foetuses in the same horn on foetal growth seems more likely to be a result of mechanical pressure arising from the distension of the uterus.