Twins: measures of birth size at different gestational ages.

Means, standard deviations, and empirical centile values are given for birth weight, length and head circumference for twins born at gestational ages 31+ to 42+ weeks. The data are compared with recent results for singletons, and the twins show a deficit that becomes greater as gestational age increases. Although weight may be slightly more affected than the other two measures, the relative growth suppression for twins is quite similar in all three dimensions. Comparatively, the average size of twins born at 33+ weeks gestational age is equal to the 36th centile for singletons; but at 40+ weeks gestational age, the average size for twins is below the 5th centile for singletons.