Mean Fetal Kidney Length at the Third Trimester: An Emerging Ultrasound Parameter for Gestational Age Assessment

Introduction Accurate gestational age (GA) determination is essential for effective obstetric care, guiding the timing of delivery, fetal evaluations, and interventions. Conventional ultrasound (USG) markers such as biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) often lose precision in the third trimester, with discrepancies of up to three weeks. These limitations highlight the need for alternative, reliable metrics. Fetal kidney length (FKL), which increases consistently by about 1 mm per week after 24 weeks of gestation and remains unaffected by growth restrictions, presents a promising alternative. This study aimed to evaluate the diagnostic utility of mean FKL for third-trimester GA assessment. Methods A cross-sectional study was conducted from March to June 2021 at a tertiary care hospital in Tamil Nadu, India. Fifty pregnant women with singleton pregnancies between 28 and 42 weeks of gestation were enrolled. Participants were required to have confirmed last menstrual period (LMP) dates and first-trimester dating scans. Pregnancies with high-risk factors, fetal anomalies, or other complications were excluded. Biometric parameters (BPD, HC, AC, and FL) and mean FKL were measured using USG. FKL was obtained by averaging the lengths of both kidneys in the paravertebral plane. FKL-based GA (FKLGA) was compared with LMP-based GA (LMPGA) and GA derived from conventional markers. Analyses included intraclass correlation coefficients (ICC), linear regression, and Bland-Altman plots. Results The mean age of participants was 23.6 ± 1.6 years, and 54% were primigravidae. The median LMPGA was 37 weeks (interquartile range (IQR): 35-39 weeks), closely aligned with FKLGA. The mean FKL was 36.3 ± 3.2 mm. Bland-Altman plots showed that FKLGA had narrower limits of agreement (LoA) (-0.5 to 1 week) compared to conventional parameters (-2 to 6 weeks). The ICC between FKLGA and LMPGA was 0.986 (95% confidence interval (CI): 0.976-0.992), significantly higher than that of conventional markers (0.539; 95% CI: 0.31-0.739). Linear regression showed that FKL explained 97.4% of GA variability (adjusted R² = 0.974), with a 1 mm increase in FKL corresponding to a one-week GA increase (β = 0.99; p < 0.001). Conventional markers explained only 57.3% of GA variability (adjusted R² = 0.573). Discussion This study underscores the reliability of FKL for third-trimester GA estimation. The strong correlation between FKLGA and LMPGA, demonstrated by high ICC and narrow limits of agreement, supports its clinical utility. Unlike conventional biometric parameters, FKL remains unaffected by growth restrictions, making it particularly valuable for late-presentation pregnancies or uncertain LMP dates. Incorporating FKL into USG protocols can address the limitations of conventional markers and improve decision-making in high-risk pregnancies. Conclusion Mean FKL is a reliable and reproducible parameter for estimating GA in the third trimester. It outperforms conventional markers and closely aligns with LMPGA, offering a robust alternative for late-pregnancy evaluations. Further research should validate its application across diverse populations and integrate it into predictive models for enhanced clinical accuracy.