Prenatal screening has advanced, offering expectant parents information about their baby’s health. Non-Invasive Prenatal Testing (NIPT) is a widely used option. Many parents ask if NIPT screens for specific chromosomal conditions like Trisomy 18, also known as Edwards Syndrome. NIPT does screen for Trisomy 18, indicating the likelihood of its presence.
How NIPT Works
NIPT is a blood test performed around 10 weeks of gestation. It analyzes small fragments of cell-free DNA (cfDNA) circulating in the mother’s bloodstream. These cfDNA fragments originate from the placenta, sharing the same genetic material as the developing baby. By examining the quantity of cfDNA from specific chromosomes, the test identifies imbalances that might indicate certain chromosomal conditions. NIPT functions as a screening test, estimating the likelihood of a condition rather than providing a definitive diagnosis.
Understanding Trisomy 18
Trisomy 18, also known as Edwards Syndrome, is a genetic condition caused by an extra copy of chromosome 18. Instead of two copies, individuals with Trisomy 18 have three copies in their cells. This additional genetic material disrupts normal development, leading to severe physical and intellectual disabilities. Babies with Trisomy 18 often experience health challenges affecting multiple organ systems, including heart defects, kidney malformations, and growth and developmental issues. The condition is associated with low birth weight and a short life expectancy, with many affected pregnancies resulting in miscarriage or stillbirth.
Detecting Trisomy 18 with NIPT
NIPT screens for Trisomy 18 by detecting an imbalance in chromosome 18 cfDNA. It identifies a higher amount of chromosome 18 genetic material in the maternal blood sample. NIPT shows high accuracy for Trisomy 18, with sensitivities (detection rates) of 96.15% to 97.4% and specificities (true negative rates) over 99.9%. While accurate, it is slightly less precise for Trisomy 18 than for Trisomy 21 (Down syndrome). A positive NIPT result for Trisomy 18 indicates an increased chance the baby has the condition, but it is not a diagnosis.
What NIPT Results for Trisomy 18 Mean
NIPT results for Trisomy 18 are reported as “low risk” or “high risk.” A “low risk” result suggests a reduced likelihood of the baby having Trisomy 18, offering reassurance. Conversely, a “high risk” result means an increased chance the baby has Trisomy 18, necessitating further evaluation.
NIPT is a screening tool, and false positives can occur, where the test indicates a high risk but the baby does not have the condition. False negatives, though rare, are also possible, meaning the test indicates a low risk when the condition is present. Understanding these nuances often involves discussion with a genetic counselor, who can help interpret the results.
Next Steps After NIPT Results
For a low-risk NIPT result, no further testing is recommended. If NIPT indicates a high risk for Trisomy 18, confirmatory diagnostic testing is the next step. The primary diagnostic tests are chorionic villus sampling (CVS) and amniocentesis. These invasive procedures involve taking a sample of placental tissue (CVS) or amniotic fluid (amniocentesis) to directly analyze the baby’s chromosomes. Genetic counseling is an important resource, providing information about the results, discussing options, and offering support to help parents make informed decisions.