The Nuchal Translucency (NT) screening uses ultrasound measurement to calculate a statistical risk for certain chromosomal abnormalities and structural issues during the first trimester. Learning that a result is outside the typical range can be concerning. A measurement considered “thickened” simply indicates an increased risk, prompting the need for further, more specific evaluation rather than providing a definitive diagnosis.
Defining the Nuchal Translucency Measurement
The nuchal translucency measurement uses ultrasound to assess the accumulation of fluid beneath the skin at the back of the developing fetus’s neck. This measurement is taken between 11 weeks and 13 weeks and six days of gestation (or when the fetus has a crown-rump length between 45 and 84 millimeters). This specific period is necessary because the fluid naturally dissipates after the 14th week, making the measurement impossible later.
A thickened measurement is generally defined as one that falls above the 95th percentile for the fetus’s gestational age. Many clinics use a threshold of 3.0 or 3.5 millimeters as a guideline for concern; for instance, an NT measurement below 2.5 millimeters at 12 weeks is typically considered normal.
The NT measurement is combined with two biochemical markers from the mother’s blood: Pregnancy-associated Plasma Protein A (PAPP-A) and human chorionic gonadotropin (hCG). This combination of ultrasound and blood tests is known as the first-trimester combined screening.
In pregnancies affected by a chromosomal condition, PAPP-A levels often appear lower than expected, while free beta-hCG levels tend to be higher. The combined data from NT thickness, PAPP-A, and hCG is mathematically integrated with maternal age to yield a calculated probability for conditions like Down syndrome.
Potential Medical Reasons for Increased Thickness
An increased NT measurement occurs due to a temporary accumulation of fluid. The causes of this fluid build-up are generally grouped into chromosomal abnormalities and structural defects. The fluid accumulation is often linked to physiological issues, such as early, transient heart failure or impaired lymphatic drainage in the neck.
A significant percentage of thickened NT cases are associated with chromosomal abnormalities, involving an incorrect number of chromosomes. The most common condition identified is Trisomy 21 (Down syndrome), which involves an extra copy of chromosome 21. Other, rarer conditions include Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome), which are often associated with more severe NT thickening.
When the fetal chromosomes are determined to be normal, a thickened NT may still indicate a structural defect elsewhere in the body. Congenital heart defects are the most frequent structural anomaly found, with the risk increasing as the NT measurement grows larger.
The impaired blood flow associated with these early cardiac issues can prevent the proper drainage of lymphatic fluid. Other potential structural issues include:
- Major organ anomalies
- Diaphragmatic hernia
- Exomphalos
- Genetic syndromes like Noonan syndrome
Follow-Up Diagnostic Procedures
Following a thickened NT result, the next step involves diagnostic procedures to provide a definitive answer. The initial follow-up often includes a Non-Invasive Prenatal Test (NIPT), which analyzes cell-free fetal DNA circulating in the mother’s blood. The NIPT offers a much higher detection rate for common trisomies than the combined screen and helps refine the risk assessment, though it remains a screening tool.
Chorionic Villus Sampling (CVS)
To confirm a chromosomal diagnosis, invasive testing is required to analyze fetal cells directly. CVS involves taking a small sample of placental tissue, usually performed between 10 and 13 weeks of gestation.
Amniocentesis
If the pregnancy is further along, Amniocentesis is offered, collecting a sample of amniotic fluid, typically after 15 weeks of gestation. Both procedures carry a small, procedure-related risk of miscarriage.
Regardless of the chromosomal results, a thickened NT necessitates specialized fetal anatomy scans for structural anomalies. A detailed ultrasound, often performed by a maternal-fetal medicine specialist around 16 to 18 weeks, examines the developing anatomy. Because of the strong link to cardiac issues, a Fetal Echocardiogram (fetal echo) is typically recommended around 20 to 22 weeks to provide an in-depth assessment of the fetal heart structure and function.
Interpreting Results and Long-Term Outlook
If the NT measurement was thickened but all follow-up tests—including NIPT, CVS or Amniocentesis, and the Fetal Echocardiogram—return normal results, the prognosis is generally favorable. The fluid accumulation often resolves completely, and these babies are typically born healthy.
The likelihood of a completely normal outcome decreases as the initial NT measurement increases. For example, when the NT is between 3.5 and 4.4 millimeters, approximately 70% of fetuses with normal chromosomes will have a normal outcome. If the measurement is 6.5 millimeters or more, the chance of a healthy outcome is significantly lower, but still possible.
When the follow-up testing identifies a specific chromosomal or structural condition, the implications are much clearer. The diagnosis allows for extensive genetic counseling, which provides detailed information about the specific condition, its severity, and available management options. This definitive information then guides the parents and their medical team in making informed decisions about the remainder of the pregnancy and planning for specialized care after birth.