This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'To say twin discordance the differences in the two twins should be: \nSelect and explain. \nA. 15% with the larger twin as index, B. 15% with the smaller twin as index, C. 25% with the larger twin as index, D. 25% with the smaller twin as index', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Unequal sizes of twin fetuses with a difference of 25% (larger twin being used as the index), is called as Discordant growth.\n\nIt is a sign of pathological growth restriction in the smaller fetus.\nAs the weight difference within a twin pair increases and the earlier the growth discordance is evident, perinatal mortality increases.\nMost common cause of discordance:\n\nIn dichorionic twins – unequal placental mass.\nIn monochorionic twins – Twin-twin transfusion syndrome/genetic syndrome.\n\nHazards of discordant growth.\n\nThe smaller fetus is at increased risk of perinatal mortality due to:\n\nRespiratory distress\nIntra ventricular hemorrhage\nSeizure\nSepsis\nPeriventricular leukomalacia\nNecrotising enterocolitis.\n\nAt a difference of more than 30%—Risk of fetal death increases.\nExtra Edge\n\nBesides weight—other ultrasonographic parameters indicating discordance:\n\n\nDifference in biparietal diameter is 6 mm or more.\nDifference in head circumference is 5% or more.\nDifference in abdominal circumference is 20 mm or more.\nOn colour Doppler ultrasound in umbilical arteries systolic to diastolic (S/D) ratio difference is 15% or more.\n\nBut amongst all criterias best is fetal weight difference i.e. more than equal to 25%.\n\nRoute of delivery: In case of discordant growth vaginal delivery is indicated if:\n\n\nCervix is ripe\nPresentation is vertex/vertex\nWeight of smaller twin is > 1500 gm.', 'role': 'assistant'}]