Can you tell me how are ultra sounds used for foetal scanning.?

Many things: Fetal ultrasound is used for size and position of fetus.Sex of fetus can sometimes be determined by about the 18th week of pregnancy. First trimester birth defects like Down syndrome and skull and spine defects.Serial scans for growth and health of fetus.Position of the fetus, placenta, and amount of amniotic fluid.Second trimester major defects such as cardiac, renal etc. Initially no. fetuses.