Discordant Twin Growth - An Overview

Introduction

Twins are at a higher risk of experiencing morbidity or mortality. The incidence of preterm birth and low birth weight is high among twins. Intrauterine growth restriction is a risk factor that affects one or both fetuses. If one fetus is more diminutive in weight or size than the other, it is termed growth discordance. Fetuses with birth weight discordance are more susceptible to morbidity.

What Is Twin Chorionicity?

The chorion is the second membrane surrounding the amniotic sac (a fluid-filled sac that protects the fetus). If there is only one chorionic membrane surrounding the amniotic sac, it is known as a monochorionic twin (Mo twins). If two chorions surround the amniotic sac, it is known as dichorionic (Di) twins. However, a combination of different amnion and chorion are present in a few twins. It includes:

• Di/Di (Dichorionic/Diamniotic) Twins: The twins are separated by their chorion and amniotic sacs.

• Mo/Di (Monochorionic/Diamniotic) Twin: The twins are in a separate amniotic sac and share the same chorion and placenta.

• Mo/Mo (Monochorionic/Monoamniotic) Twins: The twins have their own chorion and amniotic sac but share a single placenta.

What Is Discordant Twin Growth?

Discordant twin growth is used in obstetrics imaging to describe a significant difference between size and weight between two fetuses of twin pregnancies. The estimated fetal weight of the smaller twin must be lower than ten percentiles of all fetuses of the same gestational age to be considered discordant twin growth. The condition occurs in a quarter of twin pregnancies and is common in monochorionic pregnancies. Discordant twin growth can cause birth anomalies, intrauterine growth restrictions, preterm birth, infection in one of the fetuses, and death within one week of birth.

How Is Discordant Twin Growth Classified?

Discordant twin growth may be classified as:

• Weight Discordance: In this condition, the twins show a greater than 25 percent body weight difference between them. It can develop due to various factors that affect fetus growth asymmetrically or due to intrauterine growth restriction.

• Size Discordance: It is frequently associated with anemia and hematological discrepancies. This form of discordant twin growth can also develop due to twin-to-twin transfusion (unequal blood flow to twins with the same placenta).

How Is Discordant Twin Growth Calculated?

• Weight Discordance = (heavier twin weight-lighter twin weight)/heavier twin weight.

• Size or Hematocrit (HT) Discordance= (Ht at birth in the twin with a higher value-Ht at birth in the twin with a lower value)/Ht at birth in the twin with a higher value.

What Are the Risk Factors for Discordant Twin Growth?

Several maternal, fetal, and placental factors affect the development of growth discordance.

• Intrauterine growth restriction in smaller twins can restrict the fetus's development, leading to the condition. The condition is caused by unequal placental sharing.

• Placental insufficiency is a condition where the placenta does not function properly.

• Twin-twin transfusion syndrome.

• Individuals with unequal velamentous cord insertion are at a higher risk. The feature is more common in monochorionic twins.

• Individuals with single umbilical arteries are at a higher risk due to unequal blood supply between fetuses.

• Low weight gain.

• Maternal factors responsible for growth discordance are maternal hypertensive disorder, advanced maternal age, and cigarette smoking. However, the role of assisted reproductive technologies and maternal age in developing growth discordance is controversial.

• Fetal factors like monochorionic or dichorionic twins, opposite-sex of fetuses, and transplacental viral infections like cytomegalovirus affect one of the fetuses.

What Is the Role of Antenatal Ultrasound in Analyzing Discordant Twin Growth?

The ultrasound can identify various characteristics of the discordant twin growth based on the trimester of pregnancy.

• First Trimester

Early pregnancy is evaluated for discordant twin growth by using crown-rump (measurement of the length of embryo or fetus) length as a parameter. The discrepant fetal size is calculated by obtaining the difference between the crown-rump length of two fetuses followed by the division of the crown-rump length of a larger twin.

An unequal crown-rump length suggests an unequal division of cell mass in monozygotic (a single cell divides to form twins) pregnancy and differences in genetic potential between two fetuses.

• Second Trimester

Monochorionic twins must undergo an ultrasound examination every two to three weeks from 16 weeks' gestation onwards. After 24 weeks of gestation, abdominal circumference, amniotic fluid index (an indicator of fetal well-being), vertical pocket (amniotic fluid volume), umbilical artery, and dividing membrane must be recorded. The fetuses must be frequently monitored as the risk of developing complications increases. Available data from the second trimester helps classify pregnancy as high-risk or low-risk.

• Third Trimester

After 24 week gestation, ultrasound may be useful in detecting insufficient fetal growth. It is important to identify and prevent the morbidity of the smaller twin. Monochorionic twins must be monitored more frequently than dichorionic twins to avoid intrauterine death. The monochorionic twins must undergo ultrasound examination bi-weekly.

During late stages, size discordance is measured by evaluating abdominal circumference. A value greater than 20 mm is useful for diagnosing size discordant twin growth. Oligohydramnios (less amount of amniotic fluid) may be observed in fetuses with severe discordance.

1) Ultrasound Fetal Examination Modalities.

• Two-Dimensional (2D) Ultrasound Examination: Fetal growth of both twins is measured. The imaging study can also identify umbilical cord and congenital abnormalities.

• The Doppler Ultrasound: It is useful in detecting abnormalities in blood flow patterns in the fetal and placental circulations.

• A Three-Dimensional (3D) Ultrasound: The test is useful in analyzing the placenta's surface, volume, and blood flow.

How to Manage Discordant Twin Growth?

Studies suggest delivery by 37 weeks' gestation for monochorionic pregnancy and at 38 weeks gestation for dichorionic pregnancy without complications. Intrapartum management is dependent on three different fetal presentations which are:

1. Vertex-vertex is a fetal position in which both the fetal heads point downwards.

2. Vertex-non vertex is the fetal position in which one of the fetus's heads points down while the other is in a breech (the baby is lying bottom down in the uterus).

3. Both non-vertex is a fetal position in which both the fetal head is breech.

The delivery of twins is dependent on how one of the twins is presented. A normal vaginal delivery, cesarean section, or a combined procedure may be planned depending on the twin's position. Combined delivery includes both vaginal and cesarean sections and is recommended when complications like cord prolapse (occurs when the umbilical cord slips down in front of the baby after the water has broken), placental abruption (detachment of the placenta from the womb), fetal distress, and malpresentation of the second fetus develop.

What Are the Outcomes Seen in Severe Discordant Twin Growth?

In twin pregnancies, more than 20 percent discordance is associated with adverse perinatal outcomes for smaller twins. The resulting characteristic includes stillbirth, birth asphyxia (failure to establish breathing at birth), respiratory distress syndrome (a breathing disorder caused by immature lungs), neonatal infection, and birth trauma. After birth, the baby experiences severe problems.

The larger twin experiences respiratory distress syndrome at birth and occasionally premature births. However, severe complications are rare in the larger twin.

What Are the Complications That Develop From Discordant Twin Growth?

• Postpartum hemorrhage: The most common adverse complication seen in twin pregnancies.

• Intrauterine death

• Neonatal asphyxia

• Long-term neurodevelopment of smaller twins is affected.

Conclusion

Discordant twin growth is one of the factors that cause perinatal mortality and morbidity in twin pregnancy. Ultrasound examination is an essential tool for diagnosing discordant twin growth. After diagnosing fetal weight or size discrepancy, additional studies on the umbilical artery and fetal Doppler may be required to understand the fetus's condition completely. Management of discordant twins varies depending on the condition's severity and fetal position.