Dubowitz et al. have offered a scoring system for estimating the gestational age of newborn babies. If the system is applied to twin pairs, the heavier twin is generally estimated to have a greater gestational age than the lighter one. Previously this has been interpreted as a flaw in the scoring system. However, it may well be that in some twin pairs the gestational ages are slightly different and that therefore, the heavier twin would be expected to have a greater gestational age. Such cases would arise through superfecundation (the formation of two zygotes from different coitions). Superfecundation can be proved only in rare case (those with two fathers). It can be argued that the rarity of such cases is accounted for by the rarity with which women expose themselves to the risk of bearing such twins (and by the improbability of detection), rather than by the rarity of superfecundation. It is inferred that superfecundation by the same man is relatively common and that therefore dizygotic twins quite often have different gestational ages. The scoring system of Dubowitz can be tested for bias by submitting monozygotic pairs to it: the association between weight and estimated gestational age should be absent in such pairs. If the system proves free of such bias, then a finding first reported here will assume some interest: it is that in opposite-sexed twin pairs, the male is significantly more often assessed as having the greater gestational age. It is suggested that this finding should be provisionally accepted as evidence for the hyopthesis that male zygotes are formed earlier than females.