Infants with congenital malformations have on average lower birth weight than do infants without malformations. Preterm delivery and low birth weight are known to recur in sibships. The aim of the study was to compare the birth weight of siblings to malformed infants with the birth weight of infants in families without malformed infants. Data were from the Medical Birth Registry in Norway from 1967 to 1998. Births were linked to their mothers through the unique personal identification number, providing sibship files with the mother as the observation unit. The study was based on 551,478 mothers with at least two singleton infants and 209,423 mothers with at least three singletons. The authors grouped the families according to whether and in which birth order an infant had a registered congenital malformation and compared birth weight and gestational age between infants of the same birth order in families with malformations and without. Overall, in families where one or two infants had a congenital malformation, the crude and adjusted mean birth weight of nonmalformed siblings did not differ from that of infants in unaffected families, whereas it was significantly reduced for the malformed infant itself. We conclude that reduced birth weight associated with congenital anomalies is specific to the affected pregnancy.