The second trimester target scan is the gold standard fetal evaluation scan: we speak of five components in our “target scan” : a) assessment of fetal support system; b) assessment of fetal activity and behaviour; c) assessment of fetal size; d) assessment of fetal structure, the anomaly scan, and e) assessment of cervical support to know the risk of premature delivery.
Fetal support system consists of the placenta, umbilical cord, and to a certain extent, the amniotic fluid. Abnormalities in the appearance of the placenta may indicate the need for close follow up to see if the fetal growth (ie increase in size) is on par with the expected standards. Abnormality of the umbilical cord, typically single umbilical artery instead of two, also means we will have to follow up to exclude growth issues.
Fetal activity and behaviour forms an important part of the examination, since by the time of the 20-week scan there is a predictable pattern of movements in all fetuses. For example, rolling/flipping over, bending and extending the limbs, opening and closing of the hands, yawning/swallowing, breathing/hiccups, reaching out to the face etc. Normal pattern of fetal activity is reassures normal cortical development to a large extent.
Fetal Size is estimated by measurement of the baby’s head, body and thigh bone length. By combining these measurements, we will be calculating the ‘estimated’ fetal weight and compare these values to the expected standard for the gestational age.
Fetal structural assessment involves a systematic documentation of the different organs of the fetal body, both external organs, and internal organs. The protocol (standard examination) includes 3 views of the head and brain, 3 views of the spine, 3 views of the face (ears not examined routinely), 3 views of the upper limbs and lower limbs (fingers not counted routinely), 3 views of tummy organs, 6 views of the heart. In the absence of any abnormality in these standard views, the chance of malformation of any of the organs is very low.
Cervical support is assessed usually by a transvaginal (internal) scan. The cervix is the neck of the womb and has an important role in maintaining the pregnancy until full term. By meticulous examination of the cervix, its appearance, and its length we will be able to predict the chance of a preterm birth or cervical insufficiency to a large extent. Appropriate treatment will be discussed with your primary physician.
When all the 5 components are normal, one can safely reassure the couple and the risk of adverse outcome is very low.
Typically, the scan takes about 30 minutes to be completed. Not uncommonly, there will be two sessions required to allow the baby to turn over and show all the sides. The target scan is usually performed between 18 and 22 weeks.