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.
15% with the larger twin as index
15% with the smaller twin as index
25% with the larger twin as index
25% with the smaller twin as index