Nuchal translucency below the 7th percentile: a novel threshold and etiological clue for small for gestational age

Establishing a specific nuchal translucency (NT) threshold is crucial for the early detection and management of small gestational age (SGA) newborns and for elucidating the underlying pathogenic mechanisms. We aim to investigate the association between NT and SGA, and to determine whether a specific NT percentile threshold can identify distinct risk patterns for SGA. We conducted a retrospective analysis using data prospectively collected from a cohort of singleton pregnancies at 23 tertiary hospitals across 12 provinces in China between August 2017 and August 2020. The exposure was gestational age-specific NT z-score, with a focus on fetuses with low NT (< the identified threshold) in early pregnancy. The primary outcome was SGA, defined as birth weight below the 10th percentile for gestational age. SGA was further categorized as mild (≥ 3rd and < 10th percentile) and severe (< 3rd percentile). An inflection point in the association between NT z-score and SGA was observed around the 7th percentile. Unlike fetuses with normal NT (≥ the threshold), in whom NT z-score was negatively associated SGA (adjusted odds ratios [OR], 0.85; 95% confidence interval [CI] 0.80–0.92), those with low NT (< the threshold) exhibited a positive association (OR, 1.66; 95% CI 1.07–2.57). The factors associated with SGA also differed by NT status. Pre-pregnancy BMI, disparity, and family income were associated with SGA in the normal NT group, whereas maternal age was associated with SGA in the low NT group. These associations and threshold effects were observed in term births and were evident in cases of severe SGA. The association between fetal NT measured during 11–13 week gestation and SGA demonstrates a threshold effect. The pathogenesis of SGA may differ among term fetuses with different NT statuses. Trial registration Chinese Clinical Trial Registry; ChiCTR-SOC-17010976; registered 25 March 2017.