Approximately 20% of all twin pregnancies are monochorionic, with the fetuses sharing a single placenta. Although the majority of these pregnancies are uncomplicated, monochorionic twins are significantly more likely than dichorionic twins to incur complications that can threaten the life and health of one or both fetuses.

The death of one monochorionic twin leaves the other twin with a 15% risk of demise. Survival after the loss of a co-twin is also associated with a 25% incidence of neurologic injury, compared with a 2% incidence in dichorionic pregnancies. Additionally, monochorionic pregnancies carry the risk of unique complications such as twin-to-twin transfusion syndrome, selective fetal growth restriction, twin anemia polycythemia sequence, and twin reversed arterial perfusion.

Monochorionic twins should be identified in the first trimester. Chorionicity is a straightforward determination at this time but can be difficult to determine in the second trimester if a single placental mass is all that is visible on an ultrasound. Most important, however, is the need for early surveillance of monochorionic twin pregnancies and the detection of potential problems and complications.

Increased ultrasonographic surveillance recommended for monochorionic twin pregnancies has been outlined in a recent consensus statement from the North American Fetal Therapy Network (Obstet Gynecol. 2015 Jan;125[1]:118-23). Beginning at 16 weeks’ gestation, monochorionic twins should be assessed every 2 weeks using amniotic fluid balance, presence/absence of fluid within the fetal bladder, and with fetal Doppler (umbilical artery, middle cerebral artery, and ductus venosus) studies. Fetal growth should also be assessed at least every 4 weeks.

Since monochorionic twins are at increased risk for congenital heart disease, echocardiography is also performed between 18 and 22 weeks, with surveillance intervals of 2 weeks or shorter if potential complications are identified. Early detection of these and other complications allows for earlier intervention, earlier referral if necessary, and potentially better outcomes.

Twin-to-twin transfusion syndrome

Twin-to-twin transfusion syndrome (TTTS) is one of the most common and most serious complications, affecting approximately 10% of monochorionic pregnancies. Significant imbalances in blood-flow exchange lead to progressive cardiovascular decompensation that causes one twin to become a “donor” of blood volume, and the other twin to become a “recipient.” Without proper treatment between 16 and 26 weeks’ gestation, the perinatal mortality rate has been estimated to be 70% or higher.

Disease severity is classified according to the Quintero staging system. Stage I is characterized by amniotic fluid discordance. In stage II, the bladder of the donor twin is no longer visible sonographically. Stage III is marked by critically abnormal Doppler waveforms in either twin (absent/reverse end-diastolic velocity in the umbilical artery, reverse flow in the ductus venosus, or pulsatile flow in the umbilical vein). In stage IV, one of the twins has developed hydrops, and stage V is characterized by the death of one or both of the twins.

Amnioreduction to decrease intra-amniotic pressure had been the treatment of choice until a randomized controlled trial, published in 2004, demonstrated that fetoscopic laser coagulation of anastomoses was superior as a first-line treatment for severe TTTS that is diagnosed before 26 weeks. Perinatal mortality and morbidity were significantly lower after the laser treatment (N Engl J Med. 2004 Jul 8;351[2]:136-44).

Outcomes were further improved over the next decade as the laser surgery technique was modified to cover the entire vascular equator rather than selective components of the vasculature. In an open-label randomized controlled trial comparing the two approaches for severe TTTS, fetoscopic laser coagulation of the vascular equator (known as the Solomon technique) reduced the risk of twin anemia polycythemia sequence and recurrence of TTTS – the two main postoperative complications associated with residual anastomoses after selective coagulation (Lancet. 2014 Jun 21;383[9935]:2144-51).

The procedure has many challenges and can be impacted by one’s inability to see the entire vascular equator because of poor access, by the patient’s history of other interventions, and by the stage of TTTS.

Laser coagulation is regarded as the standard treatment for Quintero stage II-IV disease, and it is offered in some cases of stage I disease, such as those involving severe polyhydramnios and shortened cervix. Research currently underway is examining the outcomes of treatment for stage I disease, but data thus far suggest that intervening at stage I is generally better than expectant management.

With laser coagulation treatment, the survival rate in pregnancies complicated by TTTS is about 85%-90% for one fetus, and about 70% for both. TTTS sometimes causes one twin, particularly the recipient, to develop pulmonary valve stenosis, but this is generally a functional problem that resolves when the syndrome is treated.

After treatment, it is important to monitor for the development of twin anemia-polycythemia sequence, which may still occur if full visualization of the vascular equator was not possible or if a fine vessel was missed. Such monitoring involves weekly ultrasound surveillance with middle cerebral artery peak systolic velocity measurements.

Patients should also be monitored for abnormal neurologic development, ventriculomegaly, and other signs of abnormal brain development. Even “perfect” laser treatment with seemingly complete placental separation has been associated with abnormal neurologic development in about 10%-15% of cases.

Maternal complications with TTTS include placental abruption and preterm membrane rupture, the latter of which occurs about 15%-20% of the time.

Currently under much discussion is fetoscopic laser coagulation of TTTS placentas that have “proximate cord insertions.” The surgery in these cases – where the cords are less than 4 cm apart – is much more challenging because of technical difficulties in visualizing the vascular equator, and outcomes are being studied. Some centers will not perform laser surgery on placentas with proximate cord insertions, which fortunately are uncommon. However, the surgery is possible; I have completed three cases thus far, each with dual survival.

Selective fetal growth restriction

Selective fetal growth restriction (sFGR) stems from unequal placental sharing and affects approximately 15%-20% of all monochorionic pregnancies, making it a bit more common than TTTS. Diagnostic criteria vary, but the North American Fetal Therapy Network recommends using either an estimated fetal weight below the 10th percentile, with or without significant growth discordance (greater than 25%), or just growth discordance greater than 25%. Either provides an acceptable definition of sFGR.

With sFGR, in general, the normally growing twin has normal fluid and the growth-restricted twin has less fluid. This makes it different from TTTS, in which the twins may have different sizes but fluid discordance is always present. Also in TTTS, there is a finding of polyhydramnios in the recipient.

There are three types of sFGR, based on umbilical artery Doppler findings. In type I there is no cardiovascular imbalance, and management typically involves weekly monitoring with Doppler ultrasound. If Doppler findings remain normal for some time, monitoring every 2-4 weeks will suffice. Elective delivery is generally set for 35 or 36 weeks.

Type II sFGR involves cardiovascular compromise early in pregnancy, with umbilical artery Doppler showing persistent reversed or absent end-diastolic flow. Treatment options include monitoring closely and, in general, delivering by 32 weeks. In these cases, prematurity may jeopardize the life or health of the normally growing twin while saving the life of the growth-restricted twin.

When type II sFGR is diagnosed early, selective termination of the growth-restricted fetus may be another option. This is a relatively safe procedure overall but it carries risks such as ruptured membrane and damage to the normal twin (10%-35% risk).

Type III sFGR is uniquely unpredictable, with intermittently absent or reversed flow stemming from a large artery-artery anastomosis. The direction of blood flow may suddenly change; in fact, the diagnosis is made by placing the Doppler caliper close to the placenta cord insertion and watching the end-diastolic flow. Present, absent, and reverse flow within a minute of observation demonstrates the presence of a large artery-artery anastomosis.

The risk of unexpected fetal death with severe sFGR is estimated to be 15% or higher, and the spontaneous death of the poorly growing twin threatens both the survival and the neurologic health of the co-twin. The risk of a parenchymal lesion for the co-twin is about 20%-40%.

Management decisions can be extremely difficult. As with type II, one could manage expectantly and generally deliver by 32 weeks. Fetoscopic laser coagulation to achieve complete dichorionization, as done with TTTS, could also be discussed; this approach could save the life of one twin in the event that the co-twin dies. Finally, selective termination may again be an option. None is a perfect treatment, and parents must be thoroughly counseled and supported in understanding the options and risks.

Twin anemia polycythemia sequence

Unlike TTTS, twin anemia polycythemia sequence (TAPS) does not involve a fluid shift. Rather, red blood cells shift from one fetus to the other through extremely small-caliber vessels, leading to severe anemia of one fetus and polycythemia of the other. The chronic and unbalanced transfusion occurs in about 5% of monochorionic twins, generally after 26 weeks’ gestation.

TAPS also occurs after laser treatment for TTTS in about 10%-15% of cases (generally within 4 weeks of treatment), though this incidence is significantly reduced when complete dichorionization is achieved using the Solomon technique for fetoscopic laser coagulation. Diagnosis is made when the middle cerebral artery peak systolic velocity of the red blood cell donor is greater than 1.5 MoM and the peak systolic velocity of the recipient is less than 1.0 MoM, without amniotic fluid discordance.

There are no established preferred treatments, but fetoscopic laser coagulation is an option for some patients. Visibility can be extremely poor when TAPS occurs after a laser treatment and vessels can be difficult to identify, but in selected cases it is possible with an experienced team. When performed, treatment can be followed by delivery or by intrauterine transfusion of the anemic fetus. Intrauterine transfusion has been studied as a primary treatment, but it generally is problematic because the small vessels at the root of TAPS continue to exist.

Twin reversed arterial perfusion

In about 1% of monochorionic pregnancies, an arterial incident prevents one of the twins from developing a heart and upper body. Some research has suggested that the condition is associated with trisomies in about 10% of the cases.

The viable, structurally normal co-twin therefore acts like a pump, continually perfusing the nonviable twin through an abnormal vascular circuit that allows arterial blood to flow in a reverse direction. In the process, the normal twin, or “pump twin,” can develop heart failure and hydrops. Mortality appears to be about 55%.

Diagnosis is straightforward, but it has been challenging to determine which pregnancies will require intervention. Some research has suggested that the risk of hydrops and mortality increases significantly – and favors intervention – when the weight difference is greater than 70%. On the other hand, if the difference is less than 50%, survival of the pump twin approaches 80% and continuing surveillance may be most appropriate.

Radiofrequency ablation of the cord of the nonviable twin is one of the treatment methods and has about an 80% success rate. Another option is coagulation of the blood supply in the abnormal twin using a laser fiber via a fine needle during the first trimester. An ongoing European trial of the procedure is showing success rates of approximately 70%.

Dr. Turan is director of fetal therapy and complex obstetric surgery, and an associate professor of obstetrics, gynecology, and reproductive sciences at the University of Maryland School of Medicine, Baltimore. He reported having no relevant financial disclosures.

Detecting and managing monochorionic twin complications