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 1,070 at age 20 to about 1 in 68 at age 40 and 1 in 15 at 45. Those are the figures at 12 weeks, which is the right reference point here because the NT scan happens in the same window. Risk at term is lower than risk at 12 weeks, because a proportion of affected pregnancies miscarry between the two, so a table labelled “at term” gives noticeably smaller numbers for the same age. This calculator interpolates in log space from the published 12-week age-risk table.

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, because fetuses grow rapidly during this window, so the NT median rises with CRL.

The calculator works in three steps:

1. Estimate the median NT for the given CRL. The Fetal Medicine Foundation gives a median of 1.2 mm at a CRL of 45 mm and 1.9 mm at 84 mm, and the calculator runs a smooth curve between them. Compute the NT MoM (multiple of the median) = measured NT ÷ expected median.

2. Convert the NT MoM into a likelihood ratio (LR). Screening programmes model the logarithm of the MoM as a bell curve in each group, and this page uses the parameters Bestwick, Huttly and Wald published in 2010 from a London screening programme’s scans. In unaffected pregnancies the median is 1.0 MoM with a log10 standard deviation of 0.1275 at 11 weeks and 0.1105 at 12 to 13. In affected pregnancies the median is 2.29 MoM at 11 weeks, 1.84 at 12 and 1.47 at 13, with a standard deviation of 0.23. MoMs are capped at 2.5, and at a lower limit for each week, as the programmes do. The likelihood ratio is the height of the affected curve at your MoM divided by the height of the unaffected one.

3. Combine in odds form. Prior odds = 1 / (N − 1) for a background risk of 1 in N. Posterior odds = prior odds × LR. Posterior risk = posterior odds / (1 + posterior odds). You will often see the shortcut “posterior 1:N = background N ÷ LR” instead, and it is a good approximation while the background risk is small, but it breaks down for an older patient with a very large NT: divide 1 in 15 by a likelihood ratio of 7,000 and the shortcut reports a probability above 100%. The odds form cannot do that.

Worked example: 35-year-old patient, CRL 60 mm (12 weeks 2 days), measured NT 2.4 mm.

• Background risk at age 35: about 1:249.
• Expected median NT at CRL 60: about 1.43 mm.
• NT MoM = 2.4 ÷ 1.43 = 1.68. That is just above the 95th centile.
• LR for that MoM at 12 weeks 2 days: about 3.6.
• Posterior: about 1:69, or 1.4%.

The same patient with an NT of 1.4 mm, right on the median, comes out at about 1:1,160. A typical NT lowers the age-based risk, which surprises people.

These parameters are one programme’s. Labs fit their own medians and spreads, and they update them as scanning gets more precise, so the figure on a real screening report will differ from this one, sometimes by a factor of two.

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).
• An NT of 3.5 mm or more is above the 99th centile at every CRL and is acted on by itself: diagnostic testing and a detailed fetal heart scan are offered whatever the combined figure.
• 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.