title: Monochorionic placentas with proximate umbilical cord insertions: definition, prevalence and angio-architecture.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Division of Neonatology, Department of Pediatrics, Leiden University Medical Center, Leiden, The Netherlands. Electronic address: d.zhao@lumc.nl.
authors.forename: D P
authors.initials: DP
authors.lastname: Zhao
authors.affiliation: Division of Fetal Medicine, Department of Obstetrics, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: S H P
authors.initials: SH
authors.lastname: Peeters
authors.affiliation: Division of Fetal Medicine, Department of Obstetrics, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: J M
authors.initials: JM
authors.lastname: Middeldorp
authors.affiliation: Division of Fetal Medicine, Department of Obstetrics, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: F J
authors.initials: FJ
authors.lastname: Klumper
authors.affiliation: Division of Fetal Medicine, Department of Obstetrics, Shanghai First Maternity and Infant Hospital, Tongji University School of Medicine, Shanghai, PR China.
authors.forename: T
authors.initials: T
authors.lastname: Duan
authors.affiliation: Division of Fetal Medicine, Department of Obstetrics, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: D
authors.initials: D
authors.lastname: Oepkes
authors.affiliation: Division of Neonatology, Department of Pediatrics, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: E
authors.initials: E
authors.lastname: Lopriore
mesh_terms: D001163:Arteriovenous Anastomosis / Q000601:surgery; D001724:Birth Weight; D005260:Female; D005330:Fetofetal Transfusion / Q000175:diagnosis / Q000453:epidemiology / Q000473:pathology / Q000601:surgery; D005332:Fetoscopy / Q000706:statistics & numerical data; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D010920:Placenta / Q000473:pathology*; D010922:Placenta Diseases / Q000175:diagnosis* / Q000453:epidemiology* / Q000473:pathology*; D011247:Pregnancy; D015995:Prevalence; D014430:Twins, Monozygotic / Q000706:statistics & numerical data; D014470:Umbilical Cord / Q000473:pathology* / Q000601:surgery
keywords: Growth discordance; Monoamniotic; Monochorionic twins; Proximate cord insertions; Twin anemia-polycythemia sequence; Twin–twin transfusion syndrome