The process of monozygotic twinning in animals and humans is presented. In addition, congenital defects in monozygotic twins versus dizygotic twins and singletons is discussed. Overall, the rates of congenital anomalies are higher among whites as compared to blacks, males as compared to females, and multiples as compared to singletons. The highest rates are among infants born to women in the oldest age category and infants with birth weights below 2,000 g. Among white infants, live births in plural deliveries had 22% more congenital anomalies as compared to single live births. Among black infants, the rate was only 4% higher among multiples. It has been suggested that the more frequent occurrence of low birth weight and preterm delivery among multiple births and by race are critical to the higher incidence of congenital anomalies in twins. Newer theories relating the twinning process to congenital anomalies in twins are also presented.

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