Abundant vascular anastomoses in monoamniotic versus diamniotic monochorionic placentas

The study's aim was to compare the vascular anatomy of monoamniotic with uncomplicated diamniotic monochorionic pregnancies.

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.

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).

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.