The management of a twin pregnancy complicated by Intrauterine Growth Restriction (IUGR) involves navigating a delicate balance between the risks of prematurity and the dangers of continued growth restriction within the uterus. IUGR is defined as an estimated fetal weight below the 10th percentile for gestational age. In a twin pregnancy, this complication introduces a complex layer of risk, often involving the shared resources of the placenta and the well-being of the co-twin. Determining the optimal time for delivery requires careful, individualized assessment, as prolonging the pregnancy allows for further fetal maturation but also increases the risk of stillbirth or severe compromise due to placental failure.
Understanding Growth Restriction in Twin Pregnancies
Twin pregnancies are classified based on chorionicity (the number of placentas), which fundamentally impacts the risk profile of IUGR. Dichorionic (DC) twins have two separate placentas. They typically experience growth restriction similar to singleton pregnancies, where IUGR is usually confined to one twin due to a localized placental issue. The risk to the co-twin is less direct in DC pregnancies because their circulatory systems are independent.
Monochorionic (MC) twins share a single placenta, making them susceptible to selective IUGR (sIUGR). This occurs when the shared placenta is divided unequally, and the smaller twin receives insufficient nutrients and blood flow. Selective IUGR affects approximately 10% to 15% of MC twin pregnancies and presents a management challenge due to the vascular connections (anastomoses) between the fetuses on the placental surface.
In sIUGR, the deterioration or demise of the smaller, growth-restricted twin can cause a sudden, severe drop in blood pressure and acute blood loss in the larger twin via these shared vessels. This acute event places the surviving co-twin at significant risk of severe brain damage or death. The classification of sIUGR into types (Type I, II, or III) is based on the pattern of blood flow in the smaller twin’s umbilical artery, which guides the intensity of monitoring and the ultimate timing of intervention.
Assessing Fetal Well-Being Before Delivery
Medical professionals rely on specialized monitoring tools to assess the severity of IUGR and the immediate risk to the fetuses, which dictates the delivery timeline. Doppler velocimetry is the primary method used, which non-invasively measures blood flow patterns in various fetal vessels to evaluate placental function. Flow in the umbilical artery (UA) indicates resistance within the placenta. Absent or reversed end-diastolic flow (AEDF or REDF) signifies severely increased placental resistance and impending compromise.
The Middle Cerebral Artery (MCA) Doppler assesses blood flow resistance in the fetal brain. In response to reduced oxygen supply from placental insufficiency, the fetus attempts to redistribute blood flow, preferentially shunting oxygenated blood to the brain, an effect known as “brain sparing.” This is detected as a decrease in resistance in the MCA.
The ratio of the MCA to the UA Doppler indices, known as the Cerebroplacental Ratio (CPR), is a sensitive marker for fetal compromise. Other surveillance methods include the Non-Stress Test (NST), which records the fetal heart rate response to movement, and the Biophysical Profile (BPP). The BPP is a scoring system that uses ultrasound to evaluate four parameters along with the NST score:
- Fetal breathing
- Movement
- Tone
- Amniotic fluid volume
Medical Recommendations for Optimal Delivery Timing
The recommended timing for delivery depends heavily on the chorionicity of the twins and the specific Doppler findings. For dichorionic (DC) twins with IUGR, current guidelines suggest delivery between 36 weeks and 0 days and 37 weeks and 6 days of gestation, provided surveillance remains reassuring. If the IUGR is mild and all monitoring tests are normal, delivery may be delayed until the later end of this window.
Monochorionic (MC) twins without selective IUGR are at a higher baseline risk and are typically delivered between 36 weeks and 0 days and 36 weeks and 6 days. When sIUGR is present, the delivery window is much earlier and depends on the specific type. Type I sIUGR is characterized by continuous forward flow in the umbilical artery. Expectant management is common, with delivery planned between 34 weeks and 0 days and 36 weeks and 6 days.
Type II sIUGR involves persistently absent or reversed end-diastolic flow and carries a higher risk of unpredictable deterioration. Delivery is frequently recommended earlier, commonly between 32 weeks and 0 days and 34 weeks and 0 days, with intensive inpatient surveillance. Type III sIUGR, defined by intermittently absent or reversed flow, is the most unpredictable. Delivery may be planned between 30 weeks and 0 days and 32 weeks and 0 days, or earlier if distress emerges.
Factors Requiring Immediate Delivery
While guidelines provide an optimal window, certain acute factors necessitate an immediate delivery, regardless of gestational age. The most pressing fetal indication is acute deterioration in surveillance parameters, which signals that the fetus cannot tolerate remaining in the uterine environment. This includes the progression to persistently reversed end-diastolic flow in the umbilical artery, particularly in the sIUGR twin, or a profoundly abnormal Biophysical Profile score, such as a score of four or less.
A sudden, non-reassuring Non-Stress Test, especially one showing persistent late decelerations or minimal variability, also mandates an urgent intervention. Beyond fetal status, severe maternal complications can force an immediate delivery to protect the mother’s life, even if the fetuses are extremely preterm. Examples include severe preeclampsia with features like uncontrolled high blood pressure or liver dysfunction, HELLP syndrome, or a placental abruption, which is the premature separation of the placenta from the uterine wall.
Severe oligohydramnios, or very low amniotic fluid, in the setting of IUGR is another sign of severe placental insufficiency and often precipitates an urgent delivery. When these emergent conditions arise, the focus shifts entirely to the immediate extraction of the fetuses, accepting the risks of extreme prematurity as the lesser of two evils compared to the immediate threat of stillbirth or severe neurological injury.