First-Trimester Risk Calculator (Maternal Age + NT)

Combined first-trimester screening estimates the chance that a pregnancy is affected by trisomy 21 by combining several pieces of information: the patient’s age, the nuchal translucency (NT) ultrasound measurement, and (in fuller protocols) maternal serum biochemistry markers. This calculator uses the two ultrasound-based components: age and NT.

The starting point is the age-based background risk. The probability of trisomy 21 increases with maternal age, from roughly 1 in 1480 at age 20 to about 1 in 30 at age 40. This calculator interpolates from published age-risk tables.

NT is the fluid space at the back of the fetal neck, measured by ultrasound between 11+0 and 13+6 weeks gestation (CRL roughly 45-84 mm). Affected pregnancies tend to have larger NT than unaffected pregnancies. The exact threshold matters less than the deviation from the expected median for the gestational age — fetuses grow rapidly during this window, so the NT median rises with CRL.

The calculator works in two steps:

1. Estimate the median NT for the given CRL using a published regression. Compute the NT MoM (multiple of median) = measured NT / expected median.

2. Convert the NT MoM into a likelihood ratio (LR) using the standard log-normal distribution model: NT MoM in affected pregnancies follows log-normal with median 2.0 and standard deviation 0.6 (in log space); unaffected follows log-normal with median 1.0 and standard deviation 0.3.

3. Combine: posterior risk = (background risk × LR) / (1 + background risk × (LR − 1)). For typical small background risks, this is essentially: posterior 1:N = (background N) / LR.

Worked example: 35-year-old patient, CRL 60 mm, measured NT 2.4 mm.

• Background risk at age 35: about 1:249.
• Expected median NT at CRL 60: about 1.55 mm.
• NT MoM = 2.4 / 1.55 = 1.55.
• LR for NT MoM 1.55 ≈ 1.5.
• Posterior: 1:249 × LR 1.5 → about 1:166.

Important framing:

• This is a screening tool, not a diagnostic test. A high screening result indicates increased risk and the option to pursue diagnostic testing (CVS or amniocentesis); it does not confirm trisomy 21.
• Most positive screens are false positives. With cutoff 1:250, screen-positive rate is typically 3-5% and detection rate around 70-75% with NT alone (higher with biochemistry added).
• Modern combined first-trimester screening adds free beta-hCG and PAPP-A serum markers. Adding cell-free DNA (cfDNA) testing further improves detection above 99%, but cfDNA is itself a screening test with its own positive predictive value considerations.
• Counseling about the implications of a high-risk screen should always be done with the patient’s obstetric provider or a genetic counselor, not by interpreting an online calculator alone.

Limitations of this tool: it uses only age and NT. It does not include serum biochemistry, ductus venosus or tricuspid regurgitation findings, or fetal heart rate — all of which contribute to a complete first-trimester combined screen. Use the result as one component, not a final number.