In brief — the direct answer
Selective Fetal Growth Restriction (sFGR) occurs in identical twin pregnancies sharing a single placenta (MCDA) when the placenta is not divided equally between the two babies. One baby may have 70% of the placenta, while the other only has 30%. This leads to a significant weight difference and, more importantly, a 'shared destiny' where the health of the smaller twin can impact the survival and brain development of the larger twin. Management requires highly specialized Doppler surveillance to navigate these unique risks.
What is Selective Fetal Growth Restriction (sFGR) in Twins?
Unlike growth issues in single babies, sFGR in twins is complicated by the vascular connections (vessels) between the two babies on the shared placenta. If the smaller twin's health deteriorates, blood can suddenly 'drop' from the larger twin to the smaller one, leading to brain injury or death for both. We categorize sFGR based on the blood flow (Doppler) in the smaller twin's umbilical cord:
- Type I: Stable and regular blood flow. Generally good outcomes.
- Type II: Constantly missing or reversed blood flow (high risk).
- Type III: 'Cyclical' or unpredictable blood flow that changes second by second (hazardous).
Who is at risk?
sFGR affects about 10-15% of monochorionic twins. It is often related to where the umbilical cords insert into the placenta—if one cord is attached at the very edge (velamentous insertion), that baby is more likely to be small.
How we diagnose it
Diagnosis is made through comprehensive ultrasound:
- Weight Discordance: A gap of more than 20-25% between the twin's weights.
- Abdominal Circumference: The small twin’s belly measurement is usually below the 3rd or 10th percentile.
- Doppler Mapping: Assessing the 'plumbing' every week to check the resistance of blood flow in the brains and umbilical cords of both babies.
What does management involve?
Management depends on the type and how far along the pregnancy is:
- Expectant Monitoring (Type I): Frequent scans (weekly) with a goal of delivery around 34-35 weeks.
- Laser Surgery: In some Type II cases, we may use a laser to separate the two placental circulations, protecting the larger twin from the risks associated with the smaller one.
- Selective Reduction: In very severe cases early in pregnancy (Type II or III before 24 weeks), we may discuss the possibility of stopping the heart of the smaller twin to ensure the larger twin survives without brain damage.
- Hospitalization: Close surveillance if the smaller twin's Dopplers become critical.
What are the outcomes?
The goal is to provide the best possible outcome for at least one, and ideally both, babies:
- Type I: Over 90% survival for both babies.
- Types II & III: Higher risks of premature birth and neurological complications.
- Survival: In many cases, we can reach 32-34 weeks, where neonatal outcomes are very good.
When to see a subspecialist
If your doctor says 'one twin is smaller than the other,' you need an MFM specialist who understands the classification of sFGR and the risks of shared placental connections.
Our team, including Dr. Gareth Waring and Dr. Werner Diehl, provides the expert surveillance and surgical options needed for sFGR. We focus on protecting the healthy twin's brain while giving the smaller twin every chance to grow as safely as possible.
Questions to ask your doctor
References & Clinical Guidelines
- ISUOG Practice Guidelines: role of ultrasound in twin pregnancy (selective FGR classification)
- Consensus definition and essential reporting parameters of selective fetal growth restriction in twin pregnancy: a Delphi procedure
- Management of Monochorionic Twin Pregnancy (Green-top Guideline No. 51)