Understanding NT’s 95th Percentile:
Nuchal Translucency (NT) Thickness Measurements in the 1st Trimester and What to Do if NT is Increased?
Nuchal Translucency (NT), developed over 30 years ago in the early 1990s, remains a pivotal screening marker for evaluating early fetal well-being. Widely used, it screens for chromosomal and genetic conditions, as well as heart defects and other structural anomalies in the developing fetus. To get an understanding of NT’s 95th percentile we will discuss the importance of NT measurements, variations in cutoff values, and what to do if your baby’s NT measurement falls above the 95th centile but below the 3.5mm threshold.
The Significance of Nuchal Translucency Measurements:
NT refers to measuring the clear fluid space at the back of the baby’s neck during an ultrasound scan. According to NHS guidelines, it is measured between the 11th and 14th weeks of gestation. Presently, it is a cornerstone of the NHS Chromosomal Syndromes Screening Program conducted through Combined Screening Testing (CST). This program relies on the enduring significance of NT to assess the risk of chromosomal syndromes in the first trimester of pregnancy. Elevated NT measurements can indicate potential chromosomal abnormalities such as trisomy 21 (Down syndrome) and genetic conditions like Noonan syndrome, as well as heart defects and other structural anomalies. However, there is a significant level of uncertainty regarding what constitutes an abnormal NT measurement.
Variations in Cutoff Values:
One of the challenges with NT measurements is determining what constitutes an abnormal measurement. There is considerable variation in the cutoff values used by different research papers and guidelines. The majority of research studies and international society guidelines use the 95th centile as the threshold for abnormal NT measurements. This means that if your baby’s NT measurement falls below the 95th centile, it is considered normal. However, the NHS guidelines use a more conservative cutoff of 3.5mm. This 3.5mm cutoff is equivalent to the 99th percentile, representing a higher threshold for what is considered abnormal by NHS.
The Importance of Gestational Age:
It’s crucial to understand that the 95th centile for NT measurements varies with gestational age. Younger fetuses at 11 weeks tend to have naturally smaller NT measurements compared to those closer to 14 weeks. Therefore, determining whether your baby’s NT measurement is normal or not depends on both the absolute measurement and the gestational age.
Understanding NT’s 95th Percentile Measurements:
We understand that expectant parents might find it challenging to locate validated data concerning the 95th centile cutoff for their baby, as such data is not readily accessible online. To provide support, we have developed an NT thickness reference table based on highly esteemed international data from the study conducted by Wright et al in 20081. Additionally, we have cross-referenced data on normal NT measurements for the 10-11-week gestational period from Grande et al.’s publication in 20142, which closely aligns with Wright’s findings. Consequently, we have assembled a table that delineates the limit of an increased NT (95 centile) for each week from the 10th to the 14th week, utilising gestational age data recommended by the British Medical Ultrasound Society (2009)3.
Our chart represents the distribution of increased NT measurements (> 95th centile) in relation to the gestational age. It’s important to note that, at every stage of gestation, normal NT measurements remain below 2.8 mm. Therefore, any NT measurement equal to or greater than 2.8 mm is considered elevated.
What occurs when there is a significant elevation in NT measurements?
If NT thickness is 3.5mm or more (>99th centile) you will be referred to the fetal medicine unit (FMU) of your NHS hospital or FMU of a large regional NHS trust, where doctors will scan your baby to exclude structural anomalies associated with increased NT and offer further tests (CVS or NIPT). They will also offer fetal echocardiography at 16-20 weeks to exclude associated CHD.
What to Do if NT Measurement is Above the 95th Centile but Below 3.5mm (99th centile)?
The NHS adopts a conservative approach and does not acknowledge the 95th centile raised NT cutoff. Consequently, additional scans and tests are not scheduled for these babies before the routine 20-week anomaly scan conducted by the NHS. However, when the NT measurement surpasses the 95th percentile, it is advisable to proceed with further testing. In such instances, considering Non-invasive Prenatal Testing (NIPT) and an Early Fetal Anomaly Scan (Early Fetal Scan) are recommended.
SMART TEST – A Comprehensive Screening Option
For babies with elevated NT (between 95-99th centile), we have introduced the SMART TEST, which stands as the most advanced early reassurance package available. It comprises two expert early anomaly scans, Early Fetal Echocardiography, and the most comprehensive state-of-the-art NIPT panel. The SMART TEST can effectively rule out the majority of severe congenital heart defects (CHDs) and other structural anomalies associated with elevated NT, and reduce the probability of your baby being affected by chromosomal, genetic, or structural abnormalities.
While the SMART TEST is expensive, it is the most comprehensive early reassurance package available in the UK. For parents unable to afford the SMART TEST, there is a more affordable option of the Early Fetal Scan and an extended Panorama Test. The Early Fetal Scan screens for CHDs, but does not include a detailed examination of the fetal heart. You can further upgrade the scan and Panorama Test NIPT package to include eECHO.
If you have received concerning news about your baby’s increased nuchal translucency, we strongly recommend discussing the SMART TEST with one of our specialists.
Nuchal translucency measurements play a crucial role in assessing fetal well-being during the first trimester of pregnancy. Understanding the variations in cutoff values, gestational age, and available screening options is essential for making informed decisions if your baby’s NT measurement falls above the 95th centile but below the 3.5mm NHS threshold (99th centile). The SMART TEST, with its comprehensive NIPT panel and early fetal echocardiography, offers a valuable non-invasive screening option for parents seeking more information about their baby’s health. We are committed to offering you the best possible care, and you are always welcome to have a personal consultation with us to decide on the most suitable approach based on your individual circumstances.
(2) Grande M, Solernou R, Ferrer L, Borobio V, Jimenez JM, Bennasar M, Soler A, Borrell A. Is nuchal translucency a useful aneuploidy marker in fetuses with crown-rump length of 28-44 mm? Ultrasound Obstet Gynecol. 2014 May; 43(5):520-524