What is SIUGR?

SIUGR, which is thought to occur in approximately 10% of monochorionic twins, is defined as an estimated fetal weight (EFW) less than the 10th percentile in one twin, while the other twin has an EFW appropriate for gestational age (AGA) (10th – 90th percentile). The term SIUGR was coined by Dr. Quintero and collaborators in 2001. Since then, SIUGR has been recognized as distinct complication of monochorionic twins, i.e., in the absence of twin-twin transfusions syndrome (TTTS) or twin-anemia-polycythemia sequence (TAPS). However, patients with TTTS or TAPS may also show evidence of SIUGR.

There have been attempts to change the definition of SIUGR by including estimated fetal weight discordance and Doppler abnormalities of the SIUGR twin. However, these modifications to the original definition are unwarranted, as they decrease the sensitivity of the diagnosis.

What is the cause of SIUGR?

Monochorionic twins stem from a single embryo. When the embryo splits to result in two fetuses, the amount of placenta that each fetus gets can vary. We have termed the individual amount of placenta for each twin as “individual placental territory, or IPT.” If the IPT of one twin is such that it cannot meet the nutritional demands of the fetus, the fetus may develop SIUGR. Clinical data from our group has shown that the IPT required to result in SIUGR may be approximately <30% of the entire placental mass. The commonly used term for this situation is “Unequal placental sharing”, or UPS. Unfortunately, ultrasound is not able to determine antenatally the amount of IPT for each twin. Therefore, it is not possible to diagnosed UPS with ultrasound. SIUGR may also result from the placental vascular anastomoses. This is apparent from in-utero catch-up growth noted in some patients with SIUGR after laser occlusion of the placental vascular anastomoses. Therefore, SIUGR may result either from an IPT <30%, from placental vascular anastomoses, or both.

How can SIUGR be diagnosed?

SIUGR is diagnosed antenatally with ultrasound by showing an EFW <10th percentile in one twin and an EFW >10th percentile in the other twin. Most ultrasound reporting systems do not determine the EFW percentile before 22 weeks. Since SIUGR may occur prior to 22 weeks, we recommend using the Hadlock table, which has percentile growth starting in the first trimester. We do not recommend using EFW discordance as a proxy for the diagnosis of SIUGR as mentioned above. In rare cases, both fetuses may have an EFW <10th percentile. Such cases are called “Dual IUGR”.

Are there different types of SIUGR?

A classification of SIUGR has been proposed, based on the umbilical artery Doppler waveform of the SIUGR fetus. The classification is as follows:

• SIUGR type I: There is diastolic velocity in the umbilical artery of the SIUGR twin throughout the entire cardiac cycle.
• SIUGR type II: There is absent end-diastolic velocity in the umbilical artery (AEDV) of the SIUGR twin in every cardiac cycle.
• SIUGR type III: There is intermittent AEDV in the umbilical artery of the SIUGR twin.

The classification of SIUGR into these three types assumes that the greater the type, the worse the prognosis. This was based on the original experience of the authors of the classification, which included patients in which selective feticide was used. Our experience and that of others is that the prognosis for patients with type I or type III SIUGR is similar, and that type II SIUGR is the group associated with the worse prognosis. Unfortunately, many patients with type III SIUGR have been offered and undergone selective reduction of the SIUGR twin in other centers, an iatrogenic consequence of the wrong order of the original classification.

How can SIUGR be treated?

Depending on the gestational age at which SIUGR is diagnosed, delivery may be an option. If the diagnosis is made far from viability, different treatment options have been proposed, including:

• Expectant management. Expectant management of SIUGR may result in spontaneous demise of the SIUGR twin. Depending on the type of SIUGR, spontaneous demise of the SIUGR twin may occur in 5–30% of cases. Spontaneous demise of the IUGR twin can be associated with risk of demise of the AGA twin of approximately 30% or neurological damage to the AGA twin of approximately 20%. In our experience, patients with type I SIUGR may be followed with ultrasounds until approximately 34 weeks. Type II SIUGR patients managed expectantly have had an average gestational age at delivery of approximately 27-28 weeks. In those cases, neonatal survival of the SIUGR twin has occurred in 50% of IUGR fetuses. Expectant management may also result in development of twin-twin transfusion syndrome (TTTS). If TTTS develops, expectant management is associated with 90% risk of pregnancy loss or increased risk for neurological morbidity (from single fetal demise or from prematurity).
• Laser therapy (Selective laser photocoagulation of communicating vessels, SLPCV). The purpose of this procedure is to render the circulations of each twin independent and avoid the adverse consequences that could result from the spontaneous demise of the SIUGR twin. Typically, we offer this option to patients with type II SIUGR, and rarely to types I or III. In our experience, the likelihood of at least one fetus surviving is approximately 90%, and the likelihood of both fetuses surviving is 65-75%. The risk of neurological morbidity is 5% or less. The average gestational age at delivery is approximately 33-34 weeks. Demise of the AGA twin with survival of the SIUGR twin can theoretically occur, although we have not seen this outcome.
• Umbilical cord occlusion of the SIUGR twin. This option is associated with 90% likelihood that the co-twin survives with 5% risk of neurological damage. This option is offered in many centers. Our group does not recommend this option as a first-line choice, since ordinarily the SIUGR twin is otherwise normal and laser therapy can still allow this fetus to survive.