Selective fetal growth restriction in twins (sFGR)

Definition

• Selective fetal growth restriction is conventionally defined as an estimated fetal weight of one twin below the 10th centile with an intertwin weight discordance of at least 25%.
• By Delphi consensus, in monochorionic twins an estimated weight of one twin below the 3rd centile is sufficient; otherwise at least two of four criteria are required: estimated weight or abdominal circumference of one twin below the 10th centile, weight discordance of 25% or more, or umbilical artery pulsatility index of the smaller twin above the 95th centile.

Classification

Selective intrauterine growth restriction (sIUGR)

• Selective intrauterine growth restriction is present when one fetus is below the 10th centile or there is an estimated fetal weight discordance of approximately 25%.
• Type II is characterised by persistently absent or reversed end-diastolic flow and is associated with early deterioration.
• Type III shows intermittent absent or reversed flow due to a large network of anastomoses and carries the risk of sudden fetal death and neurological damage to the larger fetus.

• In monochorionic twins, 24 weeks of gestation separates early from late selective growth restriction.
• The Gratacós classification is based on end-diastolic flow in the umbilical artery of the smaller twin: type I positive, type II absent or reversed, type III cyclically or intermittently absent or reversed.

Aetiopathogenesis

Aetiology and pathogenesis

• The main cause is unequal sharing of placental mass and vasculature between the twins.

Clinical features

Clinical features and complications

• In type I, overall twin survival exceeds 90%.
• Type II carries a high risk of intrauterine death of the growth-restricted twin and of very preterm birth.
• In type III, there is an unpredictable 10–20% risk of sudden death of the growth-restricted fetus and neurological morbidity of up to 20% in the surviving larger twin.

Diagnosis

• The basis is ultrasound biometry of both fetuses with calculation of weight discordance and Doppler of the umbilical artery.

Keep learning in the app

Further reading (selection)

1. Ultrasound Obstet Gynecol 2025: ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy (PubMed Central)
2. DocCheck Flexikon, TRAP-Sequenz

Cross-references