Summary: A monochorionic pregnancy is a multiple pregnancy, most commonly a twin pregnancy, in which babies are dependent on a single, shared placenta. Approximately 30% of twin pregnancies in the UK are monochorionic. Monochorionic placentation can also occur in higher-order multiples.
With the increasing use of assisted reproductive technology and sociodemographic changes in our population, there has been an increase in all types of multiple pregnancies. Monochorionic and dichorionic twin pregnancies share increased risks of preterm birth, fetal growth restriction, pre-eclampsia, maternal pregnancy symptoms and postpartum haemorrhage.
The particular challenges of monochorionic pregnancies arise from the vascular placental anastomoses that are almost universal and connect the umbilical circulations of both twins. Specific complications associated with inter-twin vascular anastomoses include: twin–twin transfusion syndrome, selective growth restriction, twin anaemia-polycythaemia sequence, twin reversed arterial perfusion sequence and although not exclusive to monochorionic twin pregnancy, single intrauterine death is more common. In addition, monochorionic, monoamniotic pregnancies (1% of twin pregnancies) carry a very high risk of cord entanglement.
The purpose of this guideline is to describe and, if possible, quantify the problems associated with monochorionic placentation and to identify the best evidence to guide clinical care, including routine fetal surveillance and treatment of complications at secondary and tertiary levels.
COVID disclaimer: This guideline was developed as part of the regular programme of Green-top Guidelines, as outlined in our document Developing a Green-top Guideline: Guidance for developers (PDF), and prior to the emergence of COVID-19.
Version history: This is the second edition of this guideline; a partial update of this guideline is currently in development.
Developer declaration of interests:
Professor MD Kilby: Professor Kilby is president of the British Maternal and Fetal Medicine Society. He has received editorial fees for the commissioned book “Twins” published by Dorling Kindersley in 2013. Professor Kilby receives financial support from Siemen’s Ultrasound for training meetings held within his department.
Dr L Bricker: None declared.