An ultrasound assessment of the nuchal translucency (NT) is a standard, non-invasive screening tool offered during the first trimester of pregnancy. This scan measures a small, fluid-filled space behind the baby’s neck. An increased measurement is a statistical indicator used for risk assessment, not a definitive diagnosis. The NT scan is performed during a narrow window of time to help determine if the baby has a higher probability of having a chromosomal or structural issue.
The Nuchal Translucency Measurement
The nuchal translucency measurement is a component of the first-trimester combined screening, performed between 11 weeks and 13 weeks and six days of gestation. This timing is necessary because the fluid behind the neck is naturally reabsorbed after 14 weeks, making the measurement less accurate. The thickness of this clear space, measured in millimeters, is compared against the expected range for the baby’s crown-rump length (CRL), which reflects gestational age.
A measurement is considered “increased” when it is at or above the 95th percentile for that gestational age. The clinical threshold for triggering additional follow-up is often set at 3.0 or 3.5 millimeters. For example, 3.5 millimeters or more is often considered significantly abnormal and warrants specialist referral. The risk calculation combines the NT measurement with the mother’s age and the levels of two specific hormones in her blood: Pregnancy-Associated Plasma Protein-A (PAPP-A) and free beta-human chorionic gonadotropin (free \(\beta\)-hCG).
When a chromosomal abnormality is present, PAPP-A levels are often lower than expected, and free \(\beta\)-hCG levels can be higher, particularly in Down syndrome. Combining all three data points—NT thickness, PAPP-A, and free \(\beta\)-hCG—produces a personalized risk ratio (e.g., 1 in 100 or 1 in 1,000). This combined ratio is more accurate than relying on the NT measurement alone and serves as a statistical estimate of the likelihood of a chromosomal condition.
Conditions Associated with Increased Nuchal Translucency
An increased NT measurement is a physical marker suggesting an underlying issue, which typically falls into three categories. The most common cause for a significantly increased NT is a chromosomal abnormality (aneuploidy). This includes Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome). The likelihood of a chromosomal abnormality rises directly with the NT thickness; for instance, a thickness of 3.5 to 4.4 millimeters is associated with a chance of aneuploidy around 21%.
The second category is structural defects, particularly congenital heart defects (CHD). Even if the baby’s chromosomes are normal, an increased NT measurement raises the probability of a cardiac anomaly. This is thought to be related to delayed development of the fetal lymphatic system or early heart failure, causing fluid accumulation. The risk of a major congenital heart defect is significantly higher than in the general population, necessitating specialized follow-up.
The third category includes various genetic syndromes and structural anomalies not chromosomal in nature. Conditions like Noonan syndrome, which affects multiple body systems, are associated with increased NT even when the standard karyotype is normal. Other rare structural findings have also been reported in fetuses with an elevated NT:
- Diaphragmatic hernia
- Omphalocele (abdominal wall defect)
- Skeletal dysplasias
In many cases, the increased fluid is temporary and resolves without any identifiable cause, leading to a healthy outcome.
Next Steps After an Increased Screening Result
When first-trimester screening suggests an increased risk, the focus shifts from statistical assessment to detailed evaluation or definitive diagnosis. Non-Invasive Prenatal Testing (NIPT) is often offered as an intermediate step. NIPT analyzes cell-free DNA from the placenta circulating in the mother’s blood, providing highly accurate screening for common trisomies. However, NIPT does not evaluate all chromosomal conditions or structural defects, and it is not considered a definitive diagnosis.
To obtain a confirmed diagnosis, an invasive test is necessary to analyze the baby’s chromosomes directly. Chorionic Villus Sampling (CVS) is performed between 10 and 13 weeks of gestation by sampling placental tissue. Amniocentesis is done later, usually after 15 weeks, by sampling the amniotic fluid. Both procedures allow for detailed karyotype analysis to confirm or rule out most chromosomal abnormalities, but they carry a small risk of miscarriage. Genetic counseling is recommended before deciding on invasive testing.
Regardless of the chromosomal results, a specialized Fetal Echocardiogram is a necessary follow-up step for all cases involving an increased NT, due to the strong association with cardiac issues. This detailed ultrasound of the baby’s heart is usually performed around 18 to 22 weeks of gestation by a fetal cardiology specialist to check for structural heart defects. Cardiac issues can occur even when the chromosomal tests are normal.
Interpreting Confirmed Results and Long-Term Outlook
If definitive diagnostic tests, such as CVS or amniocentesis, return a normal karyotype, the prognosis significantly improves, though follow-up continues. For fetuses with a normal chromosomal makeup, the chance of a favorable outcome is high. Continued monitoring is recommended, especially the detailed second-trimester anatomy scan and the fetal echocardiogram, to ensure no structural issues were missed.
If follow-up scans, including the echocardiogram, also appear normal, the overall risk of a poor outcome decreases substantially. However, even with a normal karyotype and anatomy scans, a slightly elevated risk of other complications, such as preterm birth or a rare genetic syndrome, may still exist, increasing with the initial NT thickness. Genetic counseling and specialized care are necessary for managing this residual risk.
If an abnormal result is confirmed, such as a specific trisomy or structural defect, the focus shifts to informed decision-making and specialized medical care. Genetic counselors and fetal medicine specialists provide objective information about the confirmed condition, including its presentation, prognosis, and available treatment options before and after birth. They help parents understand the diagnosis and connect them with necessary support systems and specialists for management.