An ultrasound can be performed in the first trimester if menstrual dates are uncertain but in some locations it may be requested for all patients.  Because the nuchal translucency scan has a small window of time in which it can be performed, 11-14 weeks in most centers, a dating ultrasound may be done prior to first trimester genetic screening.  The dating ultrasound looks for the distance measured from the top of the baby’s head to its bottom (crown-rump measurement), and the diameter of the head is recorded – this distance is known as the biparietal diameter – the distance between the parietal bones on either side of the head.  The nuchal translucency scan is performed through the application of ultrasound targeted at the back of the baby’s neck.  The sonographer or ultrasound technician will measure the thickness of any fluid collected behind the baby’s neck.  In general, a thickness of 2-3 mm or less can be considered normal.  In the first trimester, there is an association between the size of fluid collection at the back of the fetal neck, and risk of down syndrome.  If early testing is declined, or if prenatal care starts too late, the quad screen may be the best option, given between 15-22 weeks.  The nuchal transparency scan is considered to be 80% accurate, which means there is a 20% chance of it being inaccurate (1 in 5).  If you’re offered a blood test with the scan (PAPP-A) it increases the accuracy to 85-90%.  When the nasal bone is also measured, the accuracy rises to 95%.  Speaking of blood tests, other blood tests may be offered to screen for congenital anomalies in the baby.  Along with the nuchal translucency first trimester screen, between 11-14 weeks, a blood test can be performed to measure the level of PAPP-A.  this hormone test will further help determine the risk for Down Syndrome in a fetus.

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