Clinical Significance of Nuchal Translucency Measurement in Routine Prenatal Examination
Nuchal translucency (NT) measurement is a well-established marker in prenatal screening for aneuploidy, fetal abnormalities, and other pathological conditions. Its accuracy is critical in clinical practice, as it provides valuable information for early diagnosis and management of potential fetal issues. This article delves into the clinical significance of NT measurement, its challenges, and its association with adverse pregnancy outcomes, based on a retrospective cohort study conducted at The International Peace Maternity & Child Health Hospital, affiliated with Shanghai Jiao Tong University School of Medicine.
Introduction to Nuchal Translucency Measurement
NT measurement is a first-trimester ultrasound assessment performed between 11 and 13+6 weeks of gestation. It involves measuring the fluid-filled space at the back of the fetal neck. An increased NT thickness is associated with a higher risk of chromosomal abnormalities, structural defects, and adverse pregnancy outcomes. Despite its clinical importance, obtaining accurate NT measurements can be challenging due to factors such as fetal position, sonographer expertise, and time constraints. Even in non-standard sections, NT measurement remains a valuable tool in prenatal diagnostics.
Study Design and Methodology
The study retrospectively analyzed data from singleton pregnancies screened for NT at The International Peace Maternity & Child Health Hospital between January 2013 and December 2014. The screening process combined maternal age, biochemistry (pregnancy-associated plasma protein-A and free beta-human chorionic gonadotropin), and fetal NT measurements. Ultrasound scans were primarily performed trans-abdominally by trained doctors, with trans-vaginal examinations conducted when abdominal imaging was unsatisfactory. NT measurements were included for fetuses with a crown-rump length of 45 to 84 mm.
Participants were categorized into four groups based on NT thickness: Group 1 (NT < 2.5 mm), Group 2 (NT 2.5–2.9 mm), Group 3 (NT ≥ 3.0 mm), and Group 4 (NT ≥ 3.5 mm). Fetal karyotyping was offered to women with a combined calculated risk of ≥1/270 or NT ≥ 3.0 mm. Pregnancies with raised NT but normal karyotypes were monitored with additional scans, including a detailed anatomy scan at 20–24 weeks, fetal echocardiography, and serial growth scans. Pregnancy outcomes were collected from the hospital’s database registry.
Study Population and Results
A total of 26,240 singleton pregnancies underwent NT screening during the study period. After excluding 3,039 cases lost to follow-up, 23,201 cases were included in the analysis. The mean maternal age was 29 years (range: 19–46), and the mean gestational age at the time of the scan was 11 weeks (range: 10–13+6). The population of women over 35 years old accounted for 4.19% of the cohort.
Increased NT measurements (≥2.5 mm) were observed in 1.42% (329/23,201) of cases, with 0.37% (86/23,201) having NT ≥ 3.0 mm. Among the included cases, 22,884 (98.63%) resulted in live births, while 317 (1.37%) experienced adverse outcomes, including miscarriage, termination of pregnancy (due to chromosomal abnormalities or major structural anomalies), or stillbirth.
Association Between NT Thickness and Adverse Outcomes
The study revealed a strong association between increased NT thickness and adverse pregnancy outcomes, even in euploid fetuses. The percentage of live births decreased significantly with higher NT measurements. In Group 1 (NT < 2.5 mm), 98.06% (22,429/22,872) of pregnancies resulted in live births. This percentage dropped to 44.90% (22/49) in Group 4 (NT ≥ 3.5 mm). The incidence of spontaneous abortions or fetal death in chromosomally normal fetuses increased from 0.68% (156/23,085) for NT < 3.0 mm to 4.29% (3/70) for NT ≥ 3.0 mm. In Group 4, seven cases opted for termination due to fetal edema or neck hydrocystis. Including these cases, the incidence of adverse outcomes rose to 14.29% (10/70).
Structural abnormalities were more prevalent in pregnancies with raised NT measurements. The rate of structural anomalies was 10.47% (9/86) in the NT ≥ 3.0 mm group, compared to 0.72% in the NT < 3.0 mm group. Among the nine cases with structural abnormalities, six (66.67%) were terminated.
Chromosomal Abnormalities and NT Thickness
The study also highlighted the relationship between NT thickness and chromosomal abnormalities. The rate of chromosomal abnormalities increased with NT thickness, from approximately 3.7% for NT between the 95th percentile and 3.4 mm to over 21.1% for NT ≥ 3.5 mm. Prenatal chromosome analysis was performed in 601 (2.59%) cases, with 46 (7.65%) showing abnormal karyotypes. The detection rate of abnormal chromosomes was 5.64% (30/532) for fetuses with NT < 3.0 mm and 23.19% (16/69) for those with NT ≥ 3.0 mm. The most common chromosomal abnormalities were trisomy 21 (23 cases), trisomy 18 or 13 (8 cases), and other abnormalities (15 cases). The detection rate of abnormal chromosomes differed significantly among the NT groups.
Clinical Implications and Counseling
The findings underscore the importance of NT measurement in prenatal screening and its role in identifying pregnancies at risk of adverse outcomes. Increased NT thickness, even in the absence of chromosomal abnormalities, is associated with a higher risk of structural defects, miscarriage, fetal death, fetal growth restriction, and low birth weight. These risks necessitate careful monitoring and counseling for pregnant women with raised NT measurements.
For euploid fetuses with increased NT, additional diagnostic tests, such as detailed anatomy scans and fetal echocardiography, are recommended to assess the risk of structural abnormalities. Women with high NT measurements should be informed of the potential risks and provided with comprehensive counseling to guide their decision-making process.
Challenges in NT Measurement
Despite its clinical significance, NT measurement is not without challenges. Obtaining a standard section for accurate measurement can be difficult due to factors such as fetal position, sonographer expertise, and time constraints. The study emphasizes the need for standardized protocols and continuous training for sonographers to improve the accuracy and reliability of NT measurements.
Conclusion
Nuchal translucency measurement is a crucial component of first-trimester prenatal screening, providing valuable insights into the risk of chromosomal abnormalities, structural defects, and adverse pregnancy outcomes. The study highlights the association between increased NT thickness and adverse outcomes, even in euploid fetuses, and underscores the importance of accurate measurement and comprehensive counseling. By improving the accuracy of NT measurement and enhancing prenatal care protocols, healthcare providers can better identify at-risk pregnancies and reduce the incidence of birth defects and adverse outcomes.
doi.org/10.1097/CM9.0000000000000895
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