A detailed trimester ultrasound at 18-20 weeks is part of a woman’s prenatal standard of care in order to assess fetal anatomy and growth. An 18-20 week ultrasound without soft markers or anomalies can reduce the estimated risk of Trisomy 21 (Down syndrome) by approximately 50% (Smith-Bindman, 2007; from the PGSP Guideline newly revised June 2020). Soft markers or anomalies on an 18-20 week ultrasound increase the risk for aneuploidy and should be interpreted in conjunction with the prenatal screening (SIPS, IPS, or Quad) result. For more information, see Appendix 4
of the guideline for Prenatal Screening for Down Syndrome, Trisomy 18, and Open Neural Tube Defects.
The online Trisomy 21 risk calculator below can be used to generate a new modified risk by entering your patient’s screen risk and ultrasound markers.
Disclaimer: While every attempt has been made to ensure that the information contained herein is clinically accurate and current, Perinatal Services BC acknowledges that many issues remain controversial and, therefore, may be subject to practice interpretation. The online Trisomy 21 risk calculator was developed and thoroughly tested by the Department of Medical Genetics at BC Women’s Hospital & Health Centre and further tested by PSBC. This risk calculator may help with understanding your patient’s modified risk, but it should not be used as a substitute for clinical judgment.
Note: Use Google Chrome, Mozilla Firefox, or Internet Explorer 11 to use the calculator.