Nuchal translucency (NT) screening is a common prenatal test offered during the first trimester. This ultrasound measures the fluid-filled space at the back of a baby’s neck. Its primary purpose is to evaluate the potential risk of certain congenital conditions. The NT scan is a screening tool, indicating a baby’s chance of having a condition, rather than providing a definitive diagnosis.
What is Nuchal Translucency Screening?
Nuchal translucency screening is performed within a specific timeframe: between 11 weeks and 13 weeks and 6 days of gestation. This timing is important because the fluid behind the baby’s neck is reabsorbed after 14 weeks, making accurate measurement difficult. For the scan to be effective, the baby’s crown-rump length (CRL), measured from the top of the head to the bottom of the torso, should be between 45 millimeters and 84 millimeters.
During the procedure, an ultrasound transducer is moved over the pregnant person’s abdomen, though sometimes a transvaginal approach may be used for better visualization. The sonographer captures images of the developing baby and precisely measures the fluid-filled space at the back of the neck in millimeters. This measurement helps healthcare providers assess the likelihood of specific genetic conditions.
Understanding Normal Measurements
A “normal” nuchal translucency measurement is not a fixed number, but a range varying with the baby’s gestational age and crown-rump length. As a baby grows, the fluid in this area naturally increases. An NT measurement below 3.5 millimeters is generally considered low-risk. At 12 weeks, an NT measurement is typically less than 2.5 millimeters.
The 95th percentile for a given gestational age and CRL is used as a threshold; measurements above this percentile are considered increased. For example, at 11 weeks, an NT is less than 2mm, and at 13 weeks, it’s less than 2.8mm. These values provide a guideline, but individual factors play a significant role in interpretation.
Interpreting Screening Results
The nuchal translucency measurement is combined with other factors for a comprehensive risk assessment. These factors include maternal age and results from blood tests measuring specific hormones, such as PAPP-A and free ß-hCG. This combined information helps estimate the likelihood of conditions like Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13). An increased NT measurement may also indicate a higher chance of congenital heart defects or other structural issues.
The NT scan, even when combined with blood tests, is a screening test, not a diagnostic one. A higher risk result does not mean the baby definitely has a condition; it indicates an increased statistical probability. For instance, a result of 1 in 300 suggests that for every 300 fetuses with similar measurements, only one might have a congenital condition. Many babies with an increased NT measurement are born healthy.
Following Up After Screening
After receiving NT screening results, healthcare providers discuss next steps based on the risk assessment. If screening indicates a low risk, it suggests a lower chance of the baby having the conditions screened for. In such cases, no further immediate testing is typically recommended, and the pregnancy generally proceeds with routine prenatal care.
If screening results suggest a higher risk, further evaluation is usually offered. This may involve genetic counseling to help parents understand the implications and explore options. Diagnostic tests, such as chorionic villus sampling (CVS) or amniocentesis, may be recommended to confirm or rule out a chromosomal abnormality. Non-invasive prenatal testing (NIPT), which analyzes fetal DNA from the mother’s blood, is another option that can provide more detailed risk information. An early fetal echocardiogram might also be suggested to examine the baby’s heart more closely if heart defects are a concern.