Abundant vascular anastomoses in monoamniotic diamniotic monochorionic placentas

OBJECTIVE: The study's aim was to compare the vascular anatomy of monoamniotic with uncomplicated diamniotic monochorionic pregnancies. STUDY DESIGN: The fetoplacental circulations of both twins in 18 monochorionic placentas were perfused after delivery under optimal physiologic conditions, and anastomoses were delineated by dye-contrast injection. Six were from pregnancies with monochorionic monoamniotic twins and 12 from uncomplicated monochorionic diamniotic twin pregnancies. RESULTS: The cord insertions in monochorionic monoamniotic placentas were central, with a median intercord distance of 3.2 cm (range 1.7 to 6.9 cm), whereas in monochorionic diamniotic control placentas the cord insertions (n = 24) were rarely central (marginal, 13; velamentous, 2) or eccentric (6), with a mean intercord distance of 9.6 cm (5.2 to 16.7; P < .001). Cord entanglement was present in 5 of 6 cases of monochorionic monoamniotic placentas and in none of the monochorionic diamniotic placentas. Monochorionic monoamniotic placentas had more anastomoses than did monochorionic diamniotic placentas. both overall (median 13 vs 5 respectively, P < .01) and for each of the different types (arterioarterial, venovenous, and arteriovenous, P < .01). CONCLUSIONS: Monoamniotic monochorionic placentas have significantly greater numbers of both superficial and deep anastomoses than do uncomplicated monochorionic diamniotic pregnancies, This observation suggests a vascular basis for the extreme rarity of twin-twin transfusion syndrome in monoamniotic pregnancies.