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.
- 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.
- 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
- Robinson HP, Fleming JE. (1975) A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol
- 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.
- Cuckle H. (1999) Down syndrome fetal loss rate in early pregnancy. Prenat Diagn 19,1177-1179.