Placental sharing, birthweight discordance, and vascular anastomoses in monochorionic diamniotic twin placentas

OBJECTIVE: The purpose of this study was to examine the relationship between placental territory and birthweight discordance and vascular anastomoses in monochorionic diamniotic twin placentas from pregnancies that were not complicated by twin-to-twin transfusion syndrome with 2 liveborn twins. STUDY DESIGN: Placentas originated from a prospective follow-up study of monochorionic diamniotic twins included in the first trimester. After injection with dyed barium sulphate, a digital x-ray angiography and high-resolution digital photograph were taken. The 2 venous territories were delineated on the angiogram. The diameter of each arterioarterial anastomosis and of each vein that participated in an arteriovenous anastomosis was measured on the digital photograph. Net transfusion over the arteriovenous anastomoses was calculated as the difference between the total venous diameters of the 2 placental parts. RESULTS: One hundred placentas were analyzed. Birthweight discordance increased with placental territory discordance (P <.0001). Arterioarterial diameter (P <.01), net arteriovenous transfusion (P <.001), and total anastomotic diameter (P <.01) increased with placental territory discordance. On the other hand, birthweight discordance for a given placental territory discordance decreased with increasing arterioarterial diameter (P <.01), net arteriovenous transfusion ( P <.001), and total anastomotic diameter ( P <.01). CONCLUSION: In unequally shared placentas, the 2 fetal circulations are more tightly linked than in equally shared placentas, which may reduce the birthweight discordance for a given placental territory discordance.