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Monochorionic Twins

7/1/2016 - Sharon L. Seidel, MD

Mentor:   Julie A. DeCesare, MD

Editor:  Martin E. Olsen, MD

Approved:   5/2/16

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. Nearly all are diamnionic.  Monoamniotic twinning is rare. The natural incidence is 1 in 10,000 and is increasing due to the use of assisted reproductive technology.  The risk for fetal loss 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), intrauterine growth restriction (IUGR) and discordant growth, 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.

Once a diagnosis of monochorionicity has been established, ultrasound surveillance usually begins at 16 weeks gestation and is performed every 2 weeks until delivery.  Monitoring of amniotic fluid and the presence of fluid in each twin’s bladder can help to diagnose TTTS.  Umbilical artery doppler studies and middle cerebral artery studies are crucial for diagnosing IUGR and TAPS.  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 18-22 weeks gestation. 

The monoamnionic subtype of twins carries the highest risk with perinatal mortality, up to 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.

Medical complications are more common in women with twins than singleton gestations.  Women with twins are at increased risk of hyperemesis, gestational diabetes, hypertensive disorders including preeclampsia, anemia, hemorrhage, cesarean delivery, and postpartum hemorrhage.  Management of these conditions are the same as with singleton gestations.

Further Reading:

Multifetal gestations:  twin, triplet and higher-order multifetal pregnancies.  Practice Bulletin No. 144.  American College of Obstetricians and Gynecologists.  Obstet Gynecol 2014;123:1118-32.

Moldenhauer JS, Johnson MP.  Diagnosis and management of complicated monochorionic twins.  Clin Obstet Gynecol 2015;58:632-42.

Simpson LL.  What you need to know when managing twins.  10 key facts. Obstet Gynecol Clin N Am 2015;42:225-39.

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