By Keith S. Wexler, MBA, Maternal Fetal Medicine, Prenatal Diagnosis and Biotech Consultant
Paul Wexler, M.D., F.A.C.O.G., Medical Director, GENASSIST, Inc.
Clinical Professor, Department of OB/GYN, University of Colorado Health Sciences Center
Clinical Professor, Division of Genetics/Dept. of Pediatrics, Univ. of Colorado/The Children’s Hospital
Background: The breakdown of fetal cells in the maternal circulation make it possible for laboratories to screen for Fetal DNA during pregnancy.
When fetal cells break down cell free DNA (cf DNA) is released. It is possible to duplicate many copies of this DNA so that testing is possible. This prenatal non-invasive testing can be performed as early as the 10th week of pregnancy.
Trisomy 21 (Down Syndrome) is characterized by three copies of chromosome #21 instead of the expected two copies.
If a Maternal Fetal DNA test result comes back as “High Risk” for a chromosomal abnormality, the recommendation (1) is to confirm the diagnosis by amniocentesis.
Ultrasound performed in pregnancy after 15 weeks gestation can help identify some babies with Trisomy 21.
Ultrasound Markers – Trisomy 21:
- Nuchal Fold greater than or equal to 5 mm up to 18 weeks
- Nuchal Fold greater than or equal to 6 mm from 18 to 22 weeks
- Echogenic Bowel
- Frontal Lobe greater than or equal to 2 weeks smaller than dates
- Tricerebellar Diameter greater than or equal to 2 weeks smaller than dates
- Longitudinal ear measurement more than one
- Bilateral Hydronephrosis (fluid in the kidneys) greater than or equal to 4 mm
- Missing small bone or triangular middle phalanx in the fifth digit (pinky)
- Splaying of hips greater than 90 degrees
- Big toes is spread from the rest of the toes
- Humerus greater than or equal to 2 weeks smaller than dates
- Femur greater than or equal to 2 weeks smaller than dates
(1)The American College of Obstetricians & Gynecologists, Committee Opinion Number 545 December 2012, The American College of Obstetricians and Gynecologists Committee on Genetics, The Society For Maternal-Fetal Medicine Publications Committee, “NonInvasive Prenatal Testing for Fetal Aneuploidy”.