A multigravida came for routine antenatal check up. She had a history of medical termination of pregnancy due to fetus with anencephaly. Anencephaly is best diagnosed in the present pregnancy by using:

Anencephaly is a lethal defect characterized by absence of the brain and cranium above the base of the skull and orbits. In the first half of pregnancy, the diagnosis made by elevated alpha-feto protein in amniotic fluid and confirmed by sonography. Inability to obtain a view of the biparietal diameter should raise suspicion. Hydramnios from impaired fetal swallowing is common in the third trimester. Ref: Leveno K.J., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 13. Prenatal Diagnosis and Fetal Therapy. In K.J. Leveno, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.