The impact or chorionicity on umbilical cord acid-base values in twin gestations

Objective: To evaluate the impact of chorionicity on inter-twin differences in acid-base status at birth. Methods: Records for twin pregnancies delivered at ≥ 24 weeks' gestation from 1 January 1990 to 31 June 2000 were reviewed. Collected data included maternal demographics, gestational age, fetal presentation, anesthesia, delivery mode, inter-twin interval, umbilical artery (UA) and venous (UV) acid-base values, Apgar scores and birth weights. The influence of chorionicity on umbilical cord biochemistry was evaluated. (p < 0.05 was considered significant.) Results: Analysis was carried out in 87 twin pairs (29 monochorionic, MC; and 58 dichorionic, DC). MC and DC twins were similar in maternal age (25.5 vs. 28.2 years), estimated gestational age (33.7 vs. 33.6 weeks), Cesarean delivery (55.2 vs. 52.6%), delivery interval (10 v s.5 min) and respective birth weights (twin A,1882 vs. 1981; and twin B,1828 vs. 1872 g). MC first twins had a higher UA pH (7.31 ± 0.05 vs. 7.26 ± 0.08; p = 0.0005) than DC first twins. MC first and second twins had higher UA and UV bicarbonate levels than their DC counterparts (ΔpH = 21.7 ± 5.1 vs. 18.5 ± 3.1 mmol/l and 22.0 ± 3.5 vs. 19.6 ± 2.5 mmol/l, respectively; p = 0.003). MC twins were more discordant in UA pH than DC twins (ΔpH = 0.043 ± 0.09 vs. 0.003 ± 0.07; p = 0.009). MC and DC twins had a similar venous pH (ΔpH = 0.01 ± 0.06 vs. 0.02 ± 0.06; p = 0.5). Conclusions: There is a significant association between placental chorionicity and umbilical cord biochemistry in twins. Although it is possible that the mechanism of this finding is related to placental angioarchitecture, it is unlikely to be a result of simple mixing of blood volumes between twins. The physiology of underlying processes requires further study.