1. Observations on foetal growth in human pregnancy are recorded on 22,454 single, 325 twin, 249 triplet and 27 quadruplet maternities.
2. Mean birth weights of singletons, twins, triplets and quadruplets are respectively 7·43, 5·27, 4·00 and 3·07 lb.
3. For single, twin, triplet and quadruplet pregnancies, mean durations of gestation (in days) are 280·5, 261·6, 246·8 and 236·8.
4. Mean birth weight is greater for small than for large litters even when comparison is restricted to births delivered after the same period of gestation (see Table 4).
5. It is suggested that these observations, which are in accord with reports on experimental animals, raise two questions: (a) why is foetal growth retarded in multiple pregnancy during the last weeks of gestation? and (b) why is onset of labour earlier in multiple than in single pregnancy?
6. In consideration of 5 (a) it is shown that:
(i) Mean foetal weight appears to be independent of litter size until about the 27th week of gestation.
(ii) The rate of growth of multiple foetuses is slower than that of single foetuses from a stage of gestation which varies with size of litter as follows: quadruplets from about 26 weeks; triplets from 27 weeks; and twins from 30 weeks. In the case of singletons, foetal growth is linear between 30 and 36 weeks; after 36 weeks the rate of growth is reduced.
(iii) At the times from which the respective rates of foetal growth are retarded, mean litter weights of twins, triplets and quadruplets are approximately the same. These mean litter weights (in lb.) are: quadruplets at 26 weeks, 7·6; triplets at 27 weeks, 6·9; twins at 30 weeks, 6·9. The mean weight of single births at 36 weeks is 6·5 lb.
(iv) From the time that foetal growth is retarded, litter weight increment is approximately the same for every size of litter.
Support is provided for the view that (a) retardation of foetal growth in large litters is attributable, at least in part, to crowding in the uterus (see Table 6); and (b) the restriction on foetal growth in the crowded uterus may be imposed by the placenta (see Table 7).
7. In consideration of 5 (b) it is shown that mean litter weights of singletons, twins, triplets and quadruplets at birth are respectively 7·43, 10·53, 12·00 and 12·28 lb. It is suggested that these observations are not inconsistent with the view that the early onset of labour in multiple pregnancy is due to distension of the uterus, if we suppose that the amount of distension which the uterus will tolerate during the last weeks of gestation decreases as pregnancy advances.
8. Mean litter weights of multiple births increase with birth order.