Detecting and managing monochorionic twin complications

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.

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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%.