The association of parity and birth weight: testing the sensitization hypothesis

The increase in birth weight with parity was studied in sibships of 2, 3, and 4 children using large samples from the Norwegian Birth Registry. Families with full sibs were compared to families with maternal half-sibs. The sensitization hypothesis of Warburton and Naylor predicts no increase in birth weight with parity when the mother changes mate. The hypothesis was not supported by the data, since similar increases in birth weight with parity were found in both types of families. A small effect of the sex of the first child on the birth weight of the later born children was observed.