To say twin discordance the differences in the two twins should be:

Unequal sizes of twin fetuses with a difference of 25% (larger twin being used as the index), is called as Discordant growth. It is a sign of pathological growth restriction in the smaller fetus. As the weight difference within a twin pair increases and the earlier the growth discordance is evident, perinatal mortality increases. Most common cause of discordance: In dichorionic twins – unequal placental mass. In monochorionic twins – Twin-twin transfusion syndrome/genetic syndrome. Hazards of discordant growth. The smaller fetus is at increased risk of perinatal mortality due to: Respiratory distress Intra ventricular hemorrhage Seizure Sepsis Periventricular leukomalacia Necrotising enterocolitis. At a difference of more than 30%—Risk of fetal death increases. Extra Edge Besides weight—other ultrasonographic parameters indicating discordance: Difference in biparietal diameter is 6 mm or more. Difference in head circumference is 5% or more. Difference in abdominal circumference is 20 mm or more. On colour Doppler ultrasound in umbilical arteries systolic to diastolic (S/D) ratio difference is 15% or more. But amongst all criterias best is fetal weight difference i.e. more than equal to 25%. Route of delivery: In case of discordant growth vaginal delivery is indicated if: Cervix is ripe Presentation is vertex/vertex Weight of smaller twin is > 1500 gm.