Monochorionic placentas with proximate umbilical cord insertions: definition, prevalence and angio-architecture

Not much is known on the definition, occurrence and characteristics of proximate umbilical cord insertions (PCI) in monochorionic (MC) placentas. The purpose of this study was to establish a reference range for the distance between cord insertions and to evaluate the prevalence and angio-architecture of MC placentas with PCI.

All MC placentas not treated with laser surgery were included in this study. The reference range of distance between cord insertions was created using the standard methodology proposed by Royston and Wright. We defined PCI as a cord insertion distance below the 5th centile.

A total of 369 MC placentas were analyzed during this study period. The 5th centile was calculated by the equation: 0.027 × gestational age (weeks) +2.91 (cm), and ranged from 3.3 to 4 cm throughout gestation. Accordingly, 18 of the 369 (5%) MC placentas fulfilled the definition criteria for PCI. PCI occurred frequently in MC monoamniotic placentas (53%, 9/17) but were rare in MC diamniotic placentas (3%, 9/352). The prevalence of arterio-arterial and veno-venous anastomoses in MC placentas with and without PCI was respectively 100% versus 80% (P = .12) and 56% versus 26% (P = .01). PCI may be representative of later splitting of inner cell mass.

The threshold for PCI (5th centile) is approximately 4 cm throughout gestation. PCI are rare in MC diamniotic placentas, but are quite common in MC monoamniotic placentas. MC placentas with PCI are characterized by higher rates of superficial AA and/or VV anastomoses.