Placental Characteristics of Monoamniotic Twin Pregnancies in Relation to Perinatal Outcome K.E.A. Hack a, * , M.J.C. van Gemert b , E. Lopriore c , A.H.P. Schaap d , A.J. Eggink e , S.G. Elias f , J.P.H.M. van den Wijngaard b , F.P.H.A. Vandenbussche g , J.B. Derks a , G.H.A. Visser a , P.G.J. Nikkels h a Department of Obstetrics, Wilhelmina Children’s Hospital, University Medical Center Utrecht, Lundlaan 6, 3584 EA Utrecht, The Netherlands b Laser Center, Academic Medical Center, Meibergdreef 9,1105 AZ Amsterdam, The Netherlands c Department of Neonatology, Leiden University Medical Center, Albinusdreef 2, 2333 ZA Leiden, The Netherlands d Department of Obstetrics, Academic Medical Center, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands e Department of Obstetrics, Radboud University Nijmegen Medical Center, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands f Julius Center for Health Sciences and Primary Care, University Medical Centre Utrecht, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands g Department of Obstetrics, Leiden University Medical Center, Albinusdreef 2, 2333 ZA Leiden, The Netherlands h Department of Pathology, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands article info Article history: Accepted 29 September 2008 Keywords: Monoamniotic twins Perinatal mortality Outcome Vascular anastomoses Umbilical cord insertion Placental sharing TTTS abstract Objective: To study placental characteristics in relation to perinatal outcome in 55 pairs of mono- chorionic monoamniotic (MA) twins. Methods: Between January 1998 and May 2008 55 pairs of MA twins were delivered in 4 tertiary care centers and analysed for mortality, birth weight discordancy and twin-to-twin transfusion syndrome (TTTS) in relation to type of anastomoses, type and distance between cord insertions and placental sharing. Five acardiac twins, 2 conjoined twins, 4 higher order multiples and one early termination of pregnancy were excluded, leaving 43 MA placentas for analysis. Of these 43, one placenta could not be analysed for placental vascular anastomoses due to severe maceration after single intra-uterine demise leaving 42 placentas for analysis of anastomoses. Results: Arterio-arterial (AA), venovenous (VV) and arteriovenous (AV) anastomoses were detected in 98%, 43% and 91% of MA placentas, respectively. Velamentous cord insertion was found in 4% of cases. Small distance between both umbilical cord insertions (<5 cm) was present in 53% of MA placentas. Overall perinatal loss rate was 22% (19/86). We found no association between mortality and type of anastomoses, type and distance between cord insertions and placental sharing. The incidence of TTTS was low (2%) and occurred in the only pregnancy with absent AA-anastomoses. Conclusion: Perinatal mortality in MA twins was not related to placental vascular anatomy. The almost ubiquitous presence of compensating AA-anastomoses in MA placentas appears to prevent occurrence of TTTS. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Monoamniotic (MA) twinning is a rare obstetric event, which occurs in approximately 1% of all monozygotic twin gestations. The condition is characterized by a single amniotic cavity with a single placenta and two umbilical cord insertions which are most commonly close together. MA twins are associated with high antenatal and perinatal mortality rates. Compared to older litera- ture, perinatal survival has improved substantially, from 30–70% [1–4] to 80–90% [5]. This high risk has partly been attributed to common complications in (monochorionic) twin pregnancies such as preterm delivery, low birth weight and twin–twin transfusion syndrome (TTTS), but the most important cause of death is entanglement and knotting of the umbilical cords, a complication specific to MA twins [6]. The incidence of TTTS is reported to be lower in monochorionic MA pregnancies (3–10%) [7–10] than in monochorionic diamniotic pregnancies (10–15%) [11,12]. The lower incidence of TTTS in MA twins is thought to be due to the almost universal presence of compensating arterio-arterial (AA) anastomoses in MA placentas [13]. The aim of our study was to investigate the placental angioarchitecture in a large series of MA placentas and study the * Corresponding author. Department of Obstetrics, Wilhelmina Children’s Hospital, University Medical Center Utrecht, K.E.04.123.1, P.O. Box 85090, 3508 AB Utrecht, The Netherlands. Tel.: þ31 30 2506426; fax: þ31 30 2505320. E-mail address: k.e.a.hack@umcutrecht.nl (K.E.A. Hack). Contents lists available at ScienceDirect Placenta journal homepage: www.elsevier.com/locate/placenta 0143-4004/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.placenta.2008.09.016 Placenta 30 (2009) 62–65