Birth order in delivery of twins.

A comparison between the first-born twin A and the second-born twin B was made in 103 pairs of twins delivered vaginally at the Department of Obstetrics and Gynecology, Wakayama Medical College, and at Kosaka Women's Hospital. The twins were compared for birth weight, 1- and 5-min Apgar scores, and umbilical blood gas and pH. Although the mean values showed no significant difference in birth weight between twin A and twin B, the 1- and 5-min Apgar scores were higher for twin A, and the incidence of low 1- and 5-min Apgar scores was greater for twin B. The B twins with a birth weight less than 2,500 g or delivered in the nonvertex presentation were especially at disadvantage. Blood gas analysis revealed a significant difference favoring twin A in umbilical PO2 and PCO2. The results of the present study suggest that there are differences at birth favoring the A twins despite similar perinatal mortality for both and that the B twins are at disadvantage.