This month’s open chapter is on twin pregnancy, also known as monoamniotic pregnancies. A twin pregnancy is defined as “monoamniotic” in the presence of a single placenta with one amnion and one chorion.
Monoamniotic twin pregnancies are rare, representing 0.01% of all spontaneous pregnancies, and represent a reported incidence of about 1 per 10,000 pregnancies.
The incidence of monoamniotic twin pregnancies is notably increased with in vitro fertilization (IVF), especially when embryo manipulation or assisted hatching is involved.
In this chapter you will learn more about associated anomalies. Monoamniotic pregnancies are associated with a higher risk of congenital anomalies compared to singleton, dichorionic, and monochorionic diamniotic pregnancies. The estimated prevalence of congenital anomalies in monoamniotic pregnancies ranges from 10% to 25%.
Typical complications of monoamniotic pregnancies are cord entanglement and conjoined twins. The most common anomalies in monoamniotic pregnancies include congenital cardiac defects, which account for 30% of all cases. Additionally, structural anomalies may be discordant between the two fetuses, meaning that often only one fetus is affected.
The chapter provides a thorough explanation of the diagnosis chorionicity and amnionicity, which should be performed in the first trimester scan, between 11 and 13+6 weeks of gestation, whilst also outlining the ultrasound characteristics that may suggest monoamnionicity.
While national and international guidelines lack specific management recommendations for monoamniotic twins, the chapter provides insights into ultrasound surveillance for uncomplicated monochorionic pregnancies. Additionally, it offers detailed ultrasound images that illustrate features and potential complications of monoamniotic twin pregnancies, such as cord entanglement.
For more information on prenatal diagnosis, prognosis, and management, read the VISUOG chapter on Monoamniotic Twin Pregnancies here.
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