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This study was undertaken to provide reliable up to date information on birth weights and occipitofrontal head circumference measurements in relation to gestational age for English newborn twins. Records from 36 maternity units in England, mainly from 1988-92, have provided data on birth weights for over 19,000 newborn twins with gestational ages ranging from 23 to 41 weeks, and on head circumference measurements for over 5300 twins ranging from 28 weeks to 40 weeks' gestation. Centile charts have been produced for boy and girl twins showing the distribution of these values against gestational age. The findings confirm the greater weights of boys compared with girls throughout, increasing from a mean of about 50 g at early stages to 100 g later, in a similar way to that reported for singletons. Twins were lighter than comparable singletons by about 100 g at 24 weeks, increasing progressively to 4-500 g at 38 weeks' gestation. In contrast, differences in occipitofrontal head circumferences between singletons and twins were only evident with gestations longer than 35 weeks--and from 37 weeks' gestation onwards the mean head circumference of singletons exceeded that of twins by about 5 mm. It is recommended that in evaluating the significance of the birth weight of a twin in relation to gestation, twin standards such as the ones presented here should be used rather than those relating to singletons.