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Monochorionic Twins

Author: Sharon L. Seidel, MD

Mentor: Julie A. DeCesare, MD
Editor: Sangini Sheth, MD

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The risk for fetal and neonatal mortality and long-term morbidity in monochorionic twins is higher than dichorionic twins, with a 3- fold to 4-fold increase in intrauterine demise. Placental vascular anastomoses increases the risk of intrauterine demise. Complications include twin-twin transfusion syndrome (TTTS), twin anemia-polycythemia sequence (TAPS), and twin reversed arterial perfusion (TRAP) sequence. Major structural anomalies are more frequent and can negatively impact 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.  Two placentas or differing fetal sex indicate a dichorionic pregnancy.  If only one placenta is seen, the presence of an extension of chorionic tissue into the inter-twin membrane (‘twin peak” or “lambda” sign) suggests dichorionicity.  The absence of this finding suggests monochorionicity.  The absence of an inter-twin membrane suggests monoamnionicity.

Monochorionic pregnancies are followed more closely than dichorionic pregnancies and the Society of Maternal Fetal Medicine has developed a checklist to guide management. Ultrasound surveillance begins at 16 weeks gestation and is performed every 2 weeks until delivery. his close monitoring of amniotic fluid and the presence of fluid in each twin’s bladder can help to diagnose TTTS. A detailed anatomic survey is recommended to assess for anomalies. Fetal echocardiogram should be performed at 18-22 weeks gestation because monochorionicity carries an odds ratio of almost 3.5 for congenital heart disease. Growth assessment should occur every 3 to 4 weeks and antenatal fetal surveillance is recommended to start at 32 weeks gestation

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

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

In the event of preterm labor, 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 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:

American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics, Society for Maternal-Fetal Medicine. Multifetal Gestations: Twin, Triplet, and Higher-Order Multifetal Pregnancies: ACOG Practice Bulletin, Number 231. Obstet Gynecol. 2021 Jun 1;137(6):e145-e162. doi: 10.1097/AOG.0000000000004397. PMID: 34011891.

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. Revised July 2022


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