The Calculator uses a model, based on the combination of crown-rump length, maternal age and NT thickness to estimate Down syndrome risk*. This combination is also known as "NT alone" because serum analytes are not utilized. The calculator is a screening tool, it does not provide diagnostic information.

American College of Obstetricians and Gynecologists (ACOG) Practice Bulletin Number 77, January 2007 states as a Level A recommendation: "Measurement of nuchal translucency alone is less effective for first trimester screening than is the combined test (nuchal translucency measurement and biochemical markers)."

NT alone is inferior to the combination of NT and serum screening tests in estimation of risk in singletons, and may be most useful in high-order multiple pregnancies (ie 3 or more fetuses). The results of the calculation are not a substitute for clinical judgment.

Use of the Calculator does not create any provider-patient relationship with NTQR or its committee members. Neither the Nuchal Translucency Quality Review Program, the Perinatal Quality Foundation, nor any other individual or organization party involved in the preparation or publication of this site shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any use of or reliance upon this material.

* Likelihood ratios are obtained from a Gaussian model with parameters: (1) Down syndrome mean MoMs for each completed week of gestation derived by meta-analysis¹; (2) unaffected mean MoM and standard deviation of log₁₀MoM from the target values in NTQR; and (3) unaffected standard deviation of log₁₀MoM derived by adding a factor to the unaffected variance². MoMs are calculated from NT and CRL using an NTQR equation, derived from data collected at Leeds Screening Centre, UK; completed weeks are calculated from CRL³. Maternal age-specific risk is obtained from a meta-analysis of birth prevalence⁴ and adjusted to mid-trimester⁵.

1. Cuckle H, Benn P. (2010) Multianalyte Maternal Serum Screening for Chromosomal Defects. In: Genetic Disorders and the Fetus: Diagnosis, Prevention and Treatment - 6th edition. (Ed A Milunsky & JM Milunsky), Johns Hopkins University Press Baltimore, 2010; pp 771-818.
2. Spencer K, Bindra R, Nix AB, Heath V, Nicolaides KH. (2003) Delta-NT or NT MoM, which is the most appropriate method for calculating accurate patient-specific risks for trisomy 21 in the first trimester? Ultrasound Obstet Gynecol 2003;22:142-148.
3. Robinson HP, Fleming JE. (1975) A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol 82(9), 702-10.
4. Cuckle HS, Wald NJ, Thompson SG. (1987) Estimating a woman’s risk of having a pregnancy associated with Down’s syndrome using her age and serum alpha-fetoprotein level. Br J Obstet Gynaecol 94,387-402.
5. Cuckle H. (1999) Down syndrome fetal loss rate in early pregnancy. Prenat Diagn 19,1177-1179.