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
Accurate diagnosis of anencephaly on ultrasound can be done at: September 2005 
Select and explain. 
A. 5 weeks of gestation, B. 8 weeks of gestation, C. 10 weeks of gestation, D. 14 weeks of gestation
C: 10 weeks of gestation
Answer: C: 10 weeks of gestation 
Explanation: Ans. C: 10 weeks of gestation In the first half of the pregnancy, the diagnosis is made by the elevated alfa-fetoprotein in amniotic fluid and confirmed by sonography. The findings at 13 weeks are: Absence of cranial vault Angiomatous brain tissue In the later half of the pregnancy the diagnosis is difficult specially when associated with hydramnios.