Estimate first-trimester trisomy screening context from NT measurement and maternal age, with optional nasal-bone context. This page is educational and does not reproduce a full combined-screening report.
First-trimester screening for chromosomal conditions commonly combines the nuchal translucency (NT) ultrasound measurement with maternal age and, in formal combined-screening programs, serum markers such as free β-hCG and PAPP-A. This screening window is between 11 weeks 0 days and 13 weeks 6 days gestation, when the fetal crown-rump length measures 45–84 mm.
The nuchal translucency is the sonolucent space between the skin and soft tissue overlying the cervical spine of the fetus. Increased NT thickness is associated with higher chromosomal-risk estimates and can also be associated with cardiac or other structural findings. Formal FMF-style risk reports use tightly standardized measurement technique, calibrated software, and full marker integration.
This page is a simplified educational worksheet. It uses maternal age priors plus an NT MoM-based likelihood ratio, and it shows an additional trisomy-21 context estimate when absent nasal bone is selected. Biochemistry fields are displayed for note-taking but are not incorporated into the estimate on this page.
NT screening remains an important part of first-trimester prenatal assessment because it adds structural context that serum screening and cfDNA do not fully replace. This worksheet helps place an NT value in maternal-age context, but it should not be treated as a substitute for a formal FMF-style report or specialist review.
This page uses a simplified Bayesian screening estimate. Posterior risk = (Prior × LR) / (Prior × LR + (1 − Prior)). Prior = maternal age-specific background risk. LR = Gaussian likelihood ratio from NT MoM distributions for affected vs unaffected pregnancies. NT MoM = observed NT / expected median NT for gestational age. A 1 in 150 threshold is used here as a worksheet cutoff for higher-risk screening results.
Result: T21 risk: 1 in 1,357, T18: 1 in 3,959, T13: 1 in 16,635
A 35-year-old at 12+3 weeks with NT 1.5 mm has an age-based prior trisomy-21 risk of 1 in 249. On this page, the expected NT median at that gestation is about 1.53 mm, so the NT MoM is 0.98 and the simplified likelihood ratio is about 0.18. That lowers the trisomy-21 estimate to roughly 1 in 1,357, with trisomy-18 and trisomy-13 estimates also moving into a lower-risk screening range.
The most widely used NT screening algorithm was developed by Professor Kypros Nicolaides and the Fetal Medicine Foundation in London. The FMF algorithm uses a Bayesian approach that starts with the patient's age-specific a priori risk and modifies it with likelihood ratios derived from the NT MoM value, free β-hCG MoM, and PAPP-A MoM. The likelihood ratios are calculated from the log-Gaussian distributions of these markers in affected and unaffected pregnancies, with multivariate corrections for correlations between markers. The FMF certifies individual operators and software platforms to ensure quality control.
This page does not reproduce the full FMF combined-screen report. Instead, it provides a simplified NT-centered worksheet so the age prior, NT MoM, and general risk direction remain visible in one place.
Prenatal aneuploidy screening has evolved dramatically over four decades. Second-trimester maternal serum screening (triple/quad screen) was the standard from the 1990s through the 2000s, achieving ~70% detection of trisomy 21. First-trimester combined screening (introduced in the early 2000s) improved this to ~85-90%. Cell-free DNA screening (available since 2011) now achieves >99% detection, but at higher cost and without the structural information provided by the NT scan. Current guidelines from ACOG, SMFM, and ACMG recommend offering some form of screening to all pregnant patients, with the specific approach depending on patient preferences, cost, and availability.
One of the most important but underrecognized benefits of NT screening is early detection of congenital heart defects (CHD). Fetuses with NT ≥3.5 mm and normal chromosomes have approximately a 5-7% prevalence of major CHD, compared to <1% in the general population. That association is why increased NT often leads to more detailed cardiac and structural review later in pregnancy. The 11-14 week scan can also identify other structural anomalies (anencephaly, body wall defects, megacystis) at a relatively early stage.
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This worksheet applies a simplified age-prior plus NT likelihood-ratio model to the entered screening inputs and shows the risk context in plain language. It is intentionally narrower than a full FMF combined-screen report: it does not ingest biochemistry markers into the estimate, and it should be read as screening context rather than diagnosis.
An NT measurement above the 95th percentile for gestational age (often around ≥3.5 mm) is associated with higher screening risk for trisomy 21, 18, and 13, Turner syndrome, triploidy, and other chromosomal abnormalities. Increased NT is also associated with structural anomalies including congenital heart defects, diaphragmatic hernia, skeletal dysplasias, and genetic syndromes even when chromosomes are normal. Because of that broader association, an increased NT is usually interpreted in the context of the full ultrasound and the rest of the screening pathway.
First-trimester combined screening (NT + biochemistry + maternal age) detects approximately 85-90% of trisomy 21 cases at a 5% false-positive rate. Adding nasal bone assessment increases detection to ~93%. Cell-free DNA (cfDNA/NIPT) has a detection rate of 99%+ with a 0.1% false-positive rate but is still a screening test, not a diagnosis. Definitive diagnosis is usually made with CVS (chorionic villus sampling) at 11-14 weeks or amniocentesis at 15-20 weeks, both of which carry procedure-related risks that have to be discussed in context.
cfDNA (NIPT) is more sensitive and specific for trisomy 21 specifically, but NT screening provides different information. An increased NT can point toward cardiac defects, other chromosomal abnormalities, or genetic syndromes that cfDNA does not directly assess. Many practices use one or both approaches depending on local workflow and patient preference, so this worksheet is best read as part of that broader screening context rather than as a substitute for it.
A higher-risk result on first-trimester screening means the screen is positive, not that a diagnosis has been made. Many such results are false positives. Common follow-up discussions may include genetic counseling, cfDNA as a secondary screen, CVS for first-trimester diagnosis, or amniocentesis later in pregnancy. The right next step depends on how much diagnostic certainty is needed, the ultrasound picture, and local specialist guidance.
NT measurement technique is critical for screening accuracy. Formal NT protocols call for a mid-sagittal fetal view, a neutral fetal position, and calipers placed on the inner edges of the nuchal fold lines perpendicular to the long axis. The reported value is commonly the maximum of three technically adequate images. Inter-operator variability is one of the biggest sources of error, which is why FMF-style programs use operator audit and quality control.
When NT measures ≥3.5 mm but chromosomes are normal, the pregnancy can still carry higher structural-anomaly risk. Detailed fetal echocardiography and anatomy review are commonly discussed because congenital heart defects are the most frequent association. When NT is markedly increased (for example ≥6.5 mm), the conversation may also broaden to include additional genetic testing depending on the rest of the ultrasound findings and the specialist assessment.