Diagnosis and management of monochorionic-diamniotic twins

Written by Erika Peterson, MD

Monochorionic-diamniotic twins account for approximately 20% of spontaneous twins and approximately 4% of twins conceived by artificial reproductive techniques. Confirmation of the chorionicity of twins is important as there are unique potential complications of monochorionic twins, and the management and ultrasound surveillance is different than that of dichorionic twins.

Monochorionic twins are known to have a higher risk of pregnancy complications when compared to dichorionic twins. Risks exclusive to monochorionic twins include:

1. Discordance of structural defects: The overall risk of structural defects is higher in MC twins (8%) compared to DC twins (3%) and there is a 5% risk of congenital cardiac defects. The presence of these defects can also be more complicated with monochorionic twins as there is an increased risk of complications to the normal fetus.
2. Selective intrauterine growth restriction (IUGR): Unequal placental sharing can lead to risk of IUGR in one twin. This occurs in 10-ˇ15% of monochorionic twins. Risk of fetal demise due to IUGR is more complicated due to placental vascular connections that can lead either to demise or neurologic injury of the other twin.
3. Twin-twin transfusion syndrome (TTTS): Occurs in about 10% of monochorionic twins and accounts for the majority of the morbidity and mortality of MC twins. The pathophysiology is not entirely clear, but it leads to a hemodynamic disequilibrium between the twins related to unequal placental vascular anastomosis. This leads to one twin (donor) with oligohydramnios and one twin (recipient) with polyhydramnios. This can lead to progressive hemodynamic changes in both twins, and if untreated, as high as 70-100% mortality rate.
4. Twin Reverse Arterial Perfusion (TRAP) sequence: This occurs when a viable fetus (pump twin) is perfusing a nonviable twin (acardiac) through vascular anastomosis. This can lead to heart failure in the normal twin if left untreated.
5. Twin anemia-polycythemia syndrome (TAPS): Is similar to TTTS, though it usually is a more chronic occurring condition. TAPS involves one twin developing anemia (donor) and one developing polycythemia (recipient) without a fluid discordance. It occurs spontaneously in 3-5% of MC twins, but it is more commonly seen after laser therapy for TTTS.
6. Monochorionic monoamniotic twinning: Occurs when twins share both inner and outer membrane and therefore have no dividing membrane. These cases have all of the other risks of MC twins, but they also have a significant risk of in-utero demise due to cord entanglement.

Diagnosis of chorionicity is imperative for appropriate management of twins. The ideal timing of identifying chorionicity is between 10-13 weeks. During this time, chorionicity is defined based on the presence of a dividing membrane (lack of dividing membrane is consistent with monochorionic monoamniotic), as well as presence of twin peak sign (presence of twin peak sign is diagnostic of dichorionic twins). Some complications, such as TRAP sequence and monochorionic monoamniotic twins, can be identified in this early time frame.

Given the risk of both selective IUGR as well as twin-ˇtwin transfusion syndrome with monochorionic-ˇ diamniotic twins, surveillance of this type of twin gestation is different than standard dichorionic twins. The North American Fetal Therapy Network put out the following recommendations for monitoring of uncomplicated monochorionic diamniotic twins:

1. Ultrasound surveillance at least every two weeks starting at 16 weeks.
2. Assessment of amniotic fluid maximum vertical pocket and visualization of bladder should be done at least every two weeks.
3. Growth assessment of twins at least every four weeks.
4. Fetal echocardiogram in addition to detailed anatomical survey.

If complications occur with monochorionic-diamniotic twins (growth discordance, discordant structural abnormalities or discrepancy in amniotic fluid), then referral to a Maternal Fetal Medicine  specialist is recommended. Management of these complications takes a multidisciplinary approach, and counseling about different treatment options can be best accomplished at a fetal therapy center. Treatment options, including selective reduction with cord occlusion or fetoscopic laser ablation for twin-twin transfusion syndrome, are available at many fetal therapy centers around the country, but not all options are available at every center. Many experts recommend all MC twins should be evaluated by a Maternal Fetal Medicine specialist, who can easily help coordinate referral to a fetal therapy center if complications arise.

The Fetal Concerns Center at Children’s Wisconsin provides a complete range of exceptional care for both women and high-risk pregnancies and their fetuses. Our team of experts coordinates care for all fetal conditions, ranging from initial screenings to highly complex treatments.

References:
Society for Maternal Fetal Medicine Lynn L. Simpson, Twin-twin transfusion syndrome. Amer J Obst Gynecol 2012;; 208: 3-18.
Bahtiyar, MO et al. The North American Fetal Therapy Network consensus statement: prenatal surveillance of uncomplicated monochorionic gestations.
Obstet Gynecol 2015;; 125: 118-23. Emery SP et al. The North American Fetal Therapy Network consensus statement: management of complicated monochorionic gestations. Obstet Gynecol 2015;; 126: 575-84.

The Fetal Concerns Center operates in close collaboration with Froedtert & The Medical College of Wisconsin to deliver high‐quality, comprehensive fetal care. Our providers are employed by the Medical College of Wisconsin.