Fetal oxygen and glucose utilization of uncomplicated monochorionic twins: Adapting to the intrauterine environment.

Introduction: Monochorionic twins (MC) develop under unique intrauterine conditions and show a high risk of compromise during fetal life. Here we describe umbilical vein blood flow (UVBF) and fetal oxygen and glucose utilization in uncomplicated MC twins and investigate possible differences within twin-pairs according to birth-order.

Methods: Prospective single-center study on 48 uncomplicated MC twins enrolled at the time of elective cesarean delivery. Ultrasound measurements of UVBF for Twin 1 and Twin 2 labelled according to birth-order were performed before spinal anesthesia. Umbilical arterial and venous blood samples were collected for each twin after fetal delivery, and fetal oxygen and glucose deliveries and uptakes were computed.

Results: All twins were delivered within 2 min from one-another under steady-state conditions at 36.4 weeks of median gestational age (IQR 36.0-37.0). Birthweight and umbilical cord gas analyses were within physiological ranges for all twins. Second-born twins showed significantly lower UVBF, measured before delivery, and lower median birthweight compared to first-borns. Moreover, median values of estimated fetal oxygen and glucose consumption were lower in second compared to first MC twins.

Discussion: Uncomplicated MC twins show different birthweight, oxygenation and metabolic rates based on their position in utero, hinting at pre-existing conditions possibly deriving by uneven vascular and metabolic distribution of the two placental territories. The innovative findings of this study emphasize the biological uniqueness of these pregnancies and prompt further physiological studies on MC twins and placenta metabolism.