18-20 weeks Level II scan is done for

Level II - anomaly, targeted or dating scanAt the second trimester scan they check that your baby is developing normally, they assess the gestational age by measuring the crown-rump length and will also look for any major problems. This is a routine ultrasound examination performed at 18 to 20 weeks of gestation.During the examination, the fetus is seen by abdominal ultrasound. The fetus is now large enough for an accurate survey of the fetal anatomy. With multiple pregnancies, they can be firmly diagnosed and dates and growth can also be assessed. Placental position is also determined. Fuher scans may be necessary if abnormalities are suspected(Refer: William's Obstetrics, 23rd edition)