The first trimester screening (FTS) refers to the combination of fetal ultrasound examination done at 11-14 weeks and maternal blood test. The original purpose of the FTS was to screen for Down Syndrome. By screening, we mean ‘sorting out’: that is sort out fetuses that have a higher chance of Down syndrome from those that have a lower chance. Over the past two decades, world-wide experience and expertise in to this new test has increased tremendously such that in today’s practice the purpose of this first trimester screening is actually two-fold: firstly, to assess the structural normalcy of the fetus and secondly to screen for Down syndrome.
This FTS is also referred to by different names : combined screening test (indicating a combination of scan and blood), nuchal scan (indicating the weightage given to the fetal nuchal translucency in the earlier days), NT scan (abbreviated form for nuchal translucency). Since some practitioners believe that the blood test is optional (and in some cases it is not affordable or available), it is not uncommon to perform only the scan component of the FTS. In such cases, this is usually referred as the Nuchal scan or the NT scan.
While assessing the structural normalcy of the fetus at the 11 – 14 weeks scan, a detailed protocol checking the baby’s head, face, limbs, heart, lungs, stomach, bowel, bladder, kidneys, umbilical cord, placenta, liquor is undertaken. This protocol, followed at our centre, can reliably exclude almost all lethal malformations, and 80% of all major malformations.
Combined with the test performed on the mother’s blood, this FTS detects about 90% of Down Syndrome babies.
Typically, the duration of the scan is about 20 minutes but sometimes may be extended or require more than one sitting if the fetus does not “cooperate” 🙂