3D / 4D Scans
Nuchal Translucency Scan
The NT scan or “Nuchal Translucency” scan is a targeted, specialized fetal scan that is done between 11 -14 weeks of pregnancy. More importantly, this scan achieves optimal results when the size of the fetus (crown rump length or CRL* as explained in Early pregnancy assessment) is between 45 and 84mm.
Nuchal translucency is the name for the fluid behind the neck of a fetus. An NT scan uses ultrasound to measure the amount of fluid. The fluid under the skin at the back of the baby’s neck appears “Black” on ultrasound images and is normally present in all fetuses in early pregnancy. A fetus at risk of Down syndrome tends to have a higher amount of fluid. The thickness of the fluid can predict whether the early fetus may have Down syndrome or a number of other chromosomal abnormalities. It is essential that the Nuchal scan is performed between the 11th and 14th week of pregnancy, because the accuracy is best in this period.
During this scan the fetus is seen like a “Miniature” human being and all parts of the fetus can be subjected to a preliminary assessment. This scan is also known as the first fetal anomaly scan.
3D / 4D Scans
Fetal Anomaly Scan
An important gestational window for detecting congenital malformations is between 18-20 weeks of pregnancy. Most anomalies are practically detected at this time. In India, the upper limit of termination of pregnancy is 20 weeks of gestation so it is important that we detect the fetal anomalies that merit discontinuation of pregnancy well within this limit. Most fetal care centres therefore prefer to time the second trimester scan around 18-19 weeks and this is called the “Mid Trimester Anomaly Scan.”
A detailed evaluation of fetal structure from “Head to Toe” is performed at this scan.
In addition to evaluating the structural integrity of the fetus, an important aspect of the mid trimester scan is to look for markers of fetal chromosomal abnormalities.
While most structural malformations can be detected at the mid trimester anomaly scan, a few problems may become apparent only later. Notable among these problems are microcephaly, non –lethal skeletal dysplasia’s, cerebral ventriculomegaly, congenital heart blocks etc. When such problems are diagnosed in the third trimester, it is a cause of a lot of anguish to parents and care providers alike. It is important to include parents in a “Pre-Test” counseling explaining to them that although a detailed anatomy check is undertaken, detection of structural anomalies will never be 100%.
Most parents come for an ultrasound scan assuming the fetus is normal. So, in case any abnormality is detected, it is a shock and starts a string of unexpected reactions. It is too late then to decide that they had rather not had that information at all. Hence, written and verbal information should be given to parents regarding the nature and aims of the ultrasound scan and they should be made aware of the possibility of an abnormality that may be detected on the scan.
3D / 4D Scans
Fetal Growth & Doppler Scans
Assessment of fetal growth is an important aspect of fetal evaluation. Upto 14 weeks of pregnancy, this assessment is done by measuring the CRL (crown rump length) of the fetus. After 14 weeks it becomes difficult to view the entire fetus in a single horizontal view and then fetal measurements(biometry) is done for individual parts – head circumference, abdominal circumference, long bone length etc and there are various formulae which can be used to compute the estimated fetal weight.
For every gestational age there is a range fetal weights which can be within normal limits(usually between 10th to 90th centiles).
For every gestational age there is a range fetal weights which can be within normal limits(usually between 10th to 90th centiles). At every assessment the absolute weight of the fetus is plotted on such a graph and on subsequent evaluations the pattern is compared. Fetal growth is a continuous process and there are reasons why a fetus can grow abnormally small orbig – whether the growth pattern is normal or not has to be assessed on growth scans. Ultrasound scans are accurate to about 90% in estimating fetal weight – an error of +/- 10% can be expected during such estimations.
