The 18-22 week scan is like a clinical examination of the fetus. During this scan, the head ,face,heart,lungs,stomach,bladder,kidneys and limbs of the fetus are thoroughly scanned. Fetal activity ss also observed during scanning. The umblical cord, location of the placenta, which gives nutrition to the baby and the amniotic fluid that surrounds the fetus are examined. The Length of the cervix of the uterus is measured.
In some instances, a 'Doppler study' is performed to evaluate the blood flow to various parts of the baby especially the placenta and the brain. Blood flow to other organs may be studied in indicated cases.
It is important to remember that majority of babies are normal. Birth defects are seen onlu in 1-2 % of pregnancies. In some case, there may be some minor variations observed during the scan. This should now cause anxiety and the expert can help to understand these findings better. On the other hand, some abnormalities may not be picked up on ultrasound or appear at a later time in the pregnance.
Sometimes because of the position of the baby, or the presence of scars on the mother's abdomen or the thickness of the abdominal wall, some organs may not be easily seen. In such cases you may be asked to wait/come back to scan again to see the details.