<prompt> To say twin discordance the differences in the two twins should be:15 with the larger twin as index15 with the smaller twin as index25 with the larger twin as index25 with the smaller twin as index  <response>: 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 syndromegenetic syndrome.Hazards of discordant growth.The smaller fetus is at increased risk of perinatal mortality due to:Respiratory distressIntra ventricular hemorrhageSeizureSepsisPeriventricular leukomalaciaNecrotising enterocolitis.At a difference of more than 30Risk of fetal death increases.Extra EdgeBesides weightother 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 SD 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 ripePresentation is vertexvertexWeight of smaller twin is 1500 gm. 