Prenatal screening tests offer expectant parents valuable insights into their developing baby’s health. They provide information that can help guide further decisions about care.
Components of the NT Scan and Blood Test
The Nuchal Translucency (NT) scan is a combined screening approach during pregnancy, involving a specialized ultrasound and a maternal blood test. The ultrasound measures a fluid-filled space at the back of the baby’s neck, which is a key component in assessing potential genetic variations.
The maternal blood test analyzes specific hormone levels, typically free beta-human chorionic gonadotropin (free beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A). These levels, combined with the ultrasound measurement, provide a more comprehensive risk assessment.
Conditions Screened By the Test
This combined screening assesses the likelihood of specific chromosomal conditions: Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13). Down syndrome involves an extra copy of chromosome 21, leading to intellectual disabilities and distinct physical features. Edwards syndrome (Trisomy 18) involves an extra copy of chromosome 18, often resulting in severe physical and intellectual challenges, including low birth weight, heart defects, and clenched fists. Patau syndrome (Trisomy 13) is characterized by an extra copy of chromosome 13, leading to severe physical abnormalities and intellectual disability, such as heart defects, cleft lip or palate, and extra fingers or toes.
This is a screening test, not a diagnostic one. It provides an assessment of risk, indicating a higher or lower chance of these conditions, but cannot definitively confirm or rule out a diagnosis.
How the Screening is Performed
The combined NT scan and blood test are typically performed in the first trimester, between 11 weeks and 13 weeks and 6 days of pregnancy. This timeframe allows for the most accurate measurement of the fluid behind the baby’s neck.
The NT ultrasound uses a handheld transducer on the expectant parent’s abdomen; a vaginal ultrasound may be used for clearer images. The sonographer measures the nuchal translucency (fluid-filled space behind the baby’s neck) in millimeters while the baby is in a neutral position.
A blood draw from the arm is performed and sent to a laboratory for analysis of specific hormone levels. This can be done either before or on the same day as the ultrasound.
Understanding Your Screening Results
Results from the NT scan and blood test are combined with factors like maternal age to generate a personalized risk assessment. This is typically presented as a ratio (e.g., 1 in 1000), indicating the likelihood of a condition. A “low risk” result means a reduced chance of the screened conditions.
A “high risk” result indicates an elevated likelihood of a chromosomal abnormality. This does not mean the baby definitively has a condition, but suggests a higher probability, prompting discussion about further testing options.
If a high-risk result is indicated, healthcare providers may recommend diagnostic tests like chorionic villus sampling (CVS) or amniocentesis. These procedures obtain placental tissue or amniotic fluid for definitive genetic analysis. Diagnostic tests provide a conclusive answer regarding chromosomal conditions, offering certainty that screening tests cannot. Discussing these results with a healthcare provider is important for understanding the implications for the pregnancy.