The effect of birth order in twins on fetal umbilical blood gas and apgar score.

Although recent reports have revealed no difference in perinatal mortality between a set of twins, more sensitive means other than perinatal mortality are necessary to evaluate differences in twins. This study examined differences between 34 twin pairs with respect to Apgar score, umbilical venous and arterial blood gas, and acid-base data. The parameters were measured in paired samples and compared with the paired t test and Fisher's exact probability test. The most coherent findings were a higher umbilical venous PO2 and umbilical arterial PO2 in twin A. Their differences were not due to the route of delivery, interval between twins, presentation or chorionicity. The data suggest that the second-born twin has potentially greater susceptibility to hypoxia and trauma.