The current study presents prenatal data on body, organ and placental development of monovular and diovulvar twins. Most twin measurements match mean values for single gestations through the 24th gestational week. Subsequently, growth of body and organs is not as rapid in twins as in single-born infants. Most measurements for dizygotic twins are greater than measurements for monozygotic twins at equivalent stages of gestation. Finally, a marked intrapair discordance in many body and organ measurements was found in a group of monochorionic twins with a maternal history of hydramnios. This discordance was related to the cross placental transfusion syndrome. The growth retardation evident in a group of 26 twins at birth largely disappeared by the end of the first postnatal year.