Effect of nuchal translucency thickness on pregnancy outcomes in the first trimester trisomy screening

Objective: Nuchal translucency (NT) is defined as the fluid-filled space under the skin at the back of the fetus's neck. Increased NT is indicated by measurements above 3 mm on average. As NT thickness increases, the risk of chromosomal aneuploidy also increases. The aim of our study is to evaluate the impact of increased NT thickness observed during first-trimester trisomy screening on pregnancy outcomes. Materials and Methods: In this retrospective study conducted between January 01, 2015, and December 31, 2021, on pregnant women attending the first-trimester screening at the Department of Obstetrics and Perinatology, Faculty of Medicine, Cukurova University, NT thickness was measured above 2,5 mm in 333 pregnancies. Definitive diagnoses were requested for 175 of them through chorionic villus sampling and amniocentesis. Five patients from the 175 were excluded from the study due to unattainable pregnancy outcomes. Pregnant women were divided into two groups: those with NT thickness of 2,5-3,49 mm (group 1) and those with NT thickness over 3,5 mm (group 2). The two groups were compared with each other in terms of detailed ultrasonography findings, genetic results obtained with invasive diagnosis, pregnancy outcomes (live birth, elective pregnancy termination and intrauterine dead fetus) and phenotypic characteristics. Results: Chromosomal aneuploidy was detected in 30,5% (52 pregnancies) of the 170 included in our study. Of these, 30,7% (16 pregnancies) were in Group 1, and 69,3% (36 pregnancies) were in Group 2. A statistically significant difference was found between the groups (p=0,042). Trisomy 21 was the most common chromosomal anomaly (51,9%), and it was twice as common in Group 2 compared to Group 1. Additionally, 53 pregnancies had additional ultrasound findings accompanying NT thickness. Based on the number of additional ultrasound findings, three subgroups were created: isolated NT increase, NT increase with one accompanying ultrasound finding, and NT increase with two or more accompanying ultrasound findings. No statistically significant difference was found between the groups in terms of detecting chromosomal aneuploidy (p=0,201). When individual ultrasound findings were examined, a statistically significant difference was obtained in the detection rate of chromosomal aneuploidy when nasal hypoplasia accompanied NT thickness (p<0,001). We found that the risk of chromosomal anomalies increased by 2,5 times when NT thickness was detected as 3,5 mm and above, and it increased by 5,2 times when NT thickness was accompanied by nasal hypoplasia. Regarding pregnancy outcomes, in Group 1, the rates were 69,5% for live births, 22,2% for elective termination, and 8,3% for intrauterine fetal death; in Group 2, the rates were 43,9%, 45,9%, and 10,2%, respectively. We obtained a statistically significant difference between the groups (p=0,003). Conclusion: As NT thickness increases, the risk of chromosomal aneuploidy also increases. The presence of nasal hypoplasia alongside NT thickness further increases this risk. As NT thickness increases, the likelihood of a live birth decreases while the likelihood of elective termination or intrauterine fetal death increases. Therefore, all pregnant women with increased NT thickness in the first trimester should undergo a detailed ultrasound and be informed about diagnostic genetic tests. Keywords: Nuchal translucency, NT, nasal hypoplasia, first-trimester screening test, chromosomal aneuploidy