If you are expecting identical twins who share one placenta, you may have heard of Twin-to-Twin Transfusion Syndrome (TTTS). This condition can sound frightening, but regular detailed ultrasound examinations allow specialists to detect it early and monitor your babies closely.
So, what exactly do sonographers look for during a detailed ultrasound?
What Is TTTS?
Twin-to-Twin Transfusion Syndrome (TTTS) is a complication that occurs only in monochorionic twin pregnancies, where both babies share a single placenta.
Tiny blood vessel connections within the placenta can cause blood to flow unevenly between the twins. As a result:
- The donor twin loses blood volume.
- The recipient twin receives too much blood.
Without treatment, TTTS can affect the health of both babies.
1. Confirming Chorionicity
The first step is determining whether the twins share one placenta.
TTTS develops only in monochorionic twins, making chorionicity the most important starting point of the examination.
2. Measuring Amniotic Fluid
One of the key ultrasound findings is the difference in amniotic fluid around each baby.
The sonographer measures the Deepest Vertical Pocket (DVP) for both twins.
Typical findings include:
- Donor twin: Very little amniotic fluid (oligohydramnios)
- Recipient twin: Excess amniotic fluid (polyhydramnios)
This imbalance is often the earliest sign of TTTS.
3. Evaluating the Bladder
The babies’ bladders provide important clues.
The donor twin may produce very little urine, causing the bladder to appear very small or even invisible during the examination.
The recipient twin often has a larger, well-filled bladder because increased blood flow leads to increased urine production.
4. Comparing Fetal Growth
Measurements are obtained for both babies, including:
- Biparietal Diameter (BPD)
- Head Circumference (HC)
- Abdominal Circumference (AC)
- Femur Length (FL)
- Estimated Fetal Weight (EFW)
Although growth differences do not confirm TTTS by themselves, they help distinguish TTTS from other complications such as selective fetal growth restriction (sFGR).
5. Performing Doppler Ultrasound
Blood flow is carefully assessed using Doppler ultrasound.
The following vessels are commonly evaluated:
- Umbilical Artery (UA)
- Middle Cerebral Artery (MCA)
- Ductus Venosus (DV)
- Umbilical Vein (UV)
Abnormal Doppler findings may indicate that TTTS has progressed and requires urgent evaluation.
6. Examining the Heart
The recipient twin’s heart works harder because of increased blood volume.
During the ultrasound, specialists evaluate:
- Heart size
- Cardiac function
- Valve regurgitation
- Signs of heart strain
Early detection of cardiac changes is essential for treatment planning.
7. Looking for Hydrops Fetalis
In severe cases, fluid may accumulate in different parts of the baby’s body.
The sonographer checks for:
- Ascites
- Pleural effusion
- Pericardial effusion
- Skin edema
These findings suggest advanced disease and require immediate medical attention.
What Is the Usual Ultrasound Checklist?
During a detailed ultrasound, specialists generally assess:
- Placental sharing (chorionicity)
- Amniotic fluid volume
- Fetal bladder
- Fetal growth
- Doppler blood flow
- Cardiac function
- Hydrops fetalis
Each step provides important information about the health of both babies.
Final Thoughts
TTTS is a serious but treatable complication when detected early.
Regular ultrasound surveillance is essential for monochorionic twin pregnancies because changes in amniotic fluid, blood flow, and fetal heart function may develop over time.
A detailed ultrasound is much more than simply seeing your babies’ faces—it is a comprehensive examination that helps protect both twins throughout pregnancy.
