Nuchal Thickness (NT)
In this ultrasound article you will learn about one of the most important screening tests for Trisomy 21 the nuchal thickness using ultrasound.
- Nuchal thickness (NT) is the anechoic space located behind the fetal neck that is visible and measurable in almost all fetuses from 10 to 13 weeks + 6 days.
- It is a transient marker that disappears by the end of the first trimester).
- Fetal crown-rump length must be between 38 mm and 84 mm.
- The fetus should be imaged in a sagittal section; with a magnification of the fetal image at 75% of the screen; and the amnion has to be clearly distinguished from the fetal skin.
- The measurement should be performed at the level of the maximum thickness of the subcutaneous translucency between the skin and the soft tissues overlying the cervical spine.
- The calipers must be placed “on to on” in the transonic space and the measurement taken at least 2.5 mm.
- Several are the potential causes related to the accumulation of fluid in the fetal nuchal: cardiac abnormalities and other structural defects:
- Failure or delay in the development of the lymphatic system (Turner syndrome).
- Alterations in the extracellular matrix of the skin (trisomy 21 fetuses).
- The maternal age-related (older than 35 years) risk for trisomy 21, combined with the likelihood ratio calculated from the thickness of nuchal translucency in trisomy 21 and normal fetuses, has been proposed for the screening for trisomy 21.
- In order to increase the sensitivity and reduce the false-positive rate, the risk obtained by maternal age and nuchal translucency could be combined with some maternal serum markers:
- Such as first trimester PAPP-A and free beta-human chorionic gonadotropin (hCG) levels as in the double-marker test performed in late first trimester
- At 16 weeks triple-marker test is performed (free beta-HCG , estriol (E3) and alpha fetoprotien (AFP)).
- Increased NTT, short nasal bone length (NBL) less than 50th percentile for gestational age and/or reversal of flow during atrial contraction in ductus venosus (DV) flow entails further assessment .
- Amniocentesis for fetal karyotyping or through free cell fetal DNA (NIPT) in maternal blood is mandatory to confirm diagnosis.
- If a previous Trisomy 21 or family history of Down’s syndrome were encountered, fetal karyotyping using NIPT or amniocentesis are mandatory, even if we have normal NTT, NBL, DV flow, double and triple-marker and quad-marker tests.