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Monochorionic Twins

Author: Sharon L. Seidel, MD

Mentor: Julie A. DeCesare, MD
Editor: Tiffany A. Moore Simas, MD, MPH, Med

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In the United States, twin gestation is no longer a rare event due to the increase in maternal age at conception and the increase in use of assisted reproductive technology.  Twin gestation comprises 3% of all live births; only 20% are monochorionic. Regarding amnionicity, nearly all are diamniotic; monoamniotic twinning is rare. The natural incidence of monochorionic/monoamniontic twins is 1 in 10,000; however the incidence may be increased for women undergoing IVF using zona manipulation.

The risk for fetal and neonatal mortality and long-term morbidity in monochorionic twins is significantly higher than dichorionic twins, with a 3- fold to 4-fold increase in intrauterine demise.   Shared circulation due to placental vascular anastomoses increases the risk of intrauterine demise.  Complications unique to monochorionic twins include twin-twin transfusion syndrome (TTTS), twin anemia-polycythemia sequence (TAPS), and twin reversed arterial perfusion (TRAP) sequence. Major structural anomalies are more frequently diagnosed in monochorionic twins and can negatively impact the outcome of the co-twin.

Early diagnosis of chorionicity and amnionicity is made with ultrasonography. Accuracy is highest when ultrasound is performed in the first or early second trimester.  If two placentas or differing fetal sex is seen on ultrasound, then the pregnancy must be dichorionic.  If only one placenta is seen, the presence of an extension of chorionic tissue into the inter-twin membrane suggests dichorionicity.  This is referred to as the twin peak or lambda sign.  The absence of an extension of chorionic tissue into the inter-twin membrane suggests monochorionicity.  The absence of an inter-twin membrane suggests monoamnionicity.

Given higher risk of developing complications, once a diagnosis of monochorionicity has been established, ultrasound surveillance usually begins at 16 weeks gestation and is performed every 2 weeks until delivery.  This close monitoring of amniotic fluid and the presence of fluid in each twin’s bladder can help to diagnose TTTS.  Growth assessment should occur every 3 to 4 weeks.  A detailed anatomic survey is recommended to assess for anomalies. Fetal echocardiogram should be performed at 22 weeks gestation because monochorionicity carries an odds ratio of almost 3.5 for congenital heart disease.

Monochorionic/diamniotic twins should be delivered between 34 and 37 6/7 weeks of gestation with consideration of presence/absence of complications, and risks/benefits of delivery versus expectant management. Route of delivery is determined by fetal presentation and routine obstetric factors.

The monochorionic/monoamniotic twins have a perinatal mortality risk as high as 80%, mainly due to cord entanglement.  Although optimal management remains unknown, it has become standard of care to offer inpatient management starting at 24 to 28 weeks gestation.  These patients typically undergo daily fetal surveillance, regular growth assessment, and delivery between 32 to 34 weeks gestation by cesarean.

Because preterm delivery is more likely for twins, and recommended for monoamniotic twins, antenatal corticosteroids may be considered as early as 23 weeks of gestation if delivery is anticipated within 7 days and if desired by the patient after discussion of the risks associated with periviable delivery. In the event that delivery does not occur after the first course of corticosteroids, a single dose of rescue corticosteroids may be considered if the patient is less than 34 weeks of gestation, at least 7 days have passed since the first course of corticosteroids, and delivery is anticipated within the next 7 days.

Maternal complications are similar for both monochorionic and dichorionic pregnancies, and are more common than in women with singleton gestations.  Women with twins are at increased risk of hyperemesis, gestational diabetes, hypertensive disorders including preeclampsia, anemia, cesarean delivery, and postpartum hemorrhage.  Management of these conditions is the same as with singleton gestations. Aspirin is recommended for preeclampsia prevention in all multiple gestations.

Further Reading:

Committee on Practice Bulletins—Obstetrics; Society for Maternal–Fetal Medicine.  Practice Bulletin No. 169: Multifetal Gestations: Twin, Triplet, and Higher-Order Multifetal Pregnancies. Obstet Gynecol. 2016 Oct;128(4):e131-46. doi: 10.1097/AOG.0000000000001709.

Patient Safety and Quality Committee, Society for Maternal-Fetal Medicine. Electronic address:, Hoskins IA, Combs CA. Society for Maternal-Fetal Medicine Special Statement: Updated checklists for management of monochorionic twin pregnancy. Am J Obstet Gynecol. 2020 Nov;223(5):B16-B20. doi: 10.1016/j.ajog.2020.08.066. Epub 2020 Aug 27. PMID: 32861686.

Panagiotopoulou O, Fouzas S, Sinopidis X, et al. Congenital heart disease in twins: The contribution of type of conception and chorionicity. Int J Cardiol. 2016 Sep 1;218:144-149. doi: 10.1016/j.ijcard.2016.05.029. Epub 2016 May 13.

Simpson LL.  What you need to know when managing twins: 10 key facts. Obstet Gynecol Clin North Am. 2015 Jun;42(2):225-39. doi: 10.1016/j.ogc.2015.01.002. Epub 2015 Mar 9.

Initial Approval May 2016; Revised November 2017, Revised May 2019; Revised January 2021.


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