Nuchal Translucency Calculator

Nuchal translucency, or NT, is the ultrasound appearance of fluid beneath the skin at the back of a fetus's neck. Trained sonographers measure its maximum thickness during a specific first-trimester window, usually when gestational age is 11 weeks through 13 weeks and 6 days and crown-rump length is suitable. The measurement forms one part of prenatal screening for chromosomal differences and some structural conditions. NT normally changes as a fetus grows, so a measurement should be interpreted against gestational age or crown-rump length rather than as an isolated number. This calculator provides a simple educational comparison. It estimates a median that rises from about 1.2 mm at 11 weeks to 1.8 mm at 13 weeks, then divides the entered measurement by that median to show a multiple of the median. It also groups thickness below 2.5 mm as expected, 2.5 to under 3.5 mm as increased, and 3.5 mm or more as elevated. Those categories are broad teaching aids, not diagnostic cutoffs. Screening programs use validated reference curves, strict image-quality criteria, exact crown-rump length, maternal age, blood markers, pregnancy history, and sometimes cell-free DNA testing. Maternal age is collected here to emphasize that background probability changes with age, but the calculator deliberately does not invent an individualized trisomy probability. Only an accredited screening system with appropriate population data should report such a risk. An increased NT measurement does not mean that a fetus has a chromosomal or structural condition. Many pregnancies with increased NT have healthy outcomes. Conversely, an NT within the expected range cannot exclude all conditions. Clinicians may discuss detailed ultrasound, noninvasive prenatal testing, genetic counseling, chorionic villus sampling, or amniocentesis depending on the complete screening result, gestational timing, medical history, and patient preferences. Accurate measurement depends on fetal position, image magnification, caliper placement, and sonographer certification. Always use the value and gestational age printed on the formal ultrasound report rather than estimating from an image. This calculator is intended to explain how gestational context and multiples of the median work. It cannot replace prenatal care, validated risk software, diagnostic testing, or counseling from an obstetric or maternal-fetal medicine professional.