New assessment of the effects of birth order and socioeconomic status on birth weight.

A survey of the 20 698 singleton births occurring in one year to women resident in the Greater Dublin area provided information on birth weight, birth order, and social class. Low (less than or equal to 2500 g), suboptimal (less than or equal to 3000 g), and optimal (3001-4499 g) birth weights all showed a linear relation with social class. The incidence of low and suboptimal birth weight was highest in first, fifth, and subsequent births, and conversely optimal weight was commonest in second, third, and fourth births. Analysis indicated that a major part of the birth-order effect was attributable to social class. Birthweight categories give information which may be distorted when using mean weight alone. The ue of suboptimal and optimal weight offers the possibility of more accurate assessment of trends in performance, particularly in small samples, than does the conventional sole use of low birth weight. Low and suboptimal birth weights are uncommon in Dublin.