Mean Fetal Kidney Length: An Underutilized Ultrasound Parameter for Gestational Age Assessment in Third-Trimester Pregnancy—A Case Series

Abstract

Conventional third-trimester biometric parameters- biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL carry a margin of error up to three weeks in gestational age (GA) estimation, and are further confounded by intrauterine growth restriction (IUGR). Mean fetal kidney length (mFKL) increases linearly at approximately 1 mm per gestational week after 24 weeks and is unaffected by fetal growth restriction, offering a reliable surrogate. To evaluate the diagnostic utility of mFKL for third-trimester GA estimation in (i) IUGR fetuses and (ii) pregnancies with unknown or uncertain last menstrual period (LMP), with clinical case illustrations. Four representative third-trimester cases are presented: (1) overt DM with single umbilical artery and late-presenting biometry discordance; (2) suspected IUGR with significant biometry-LMP discordance; (3) late-presenting patient with unknown LMP; and (4) IUGR with Doppler evidence of placental insufficiency. In all cases, mFKL (mean of left and right kidney lengths in paravertebral plane) showed superior concordance with confirmed or estimated true GA compared to conventional biometric composite. An mFKL of 34.4 mm (mean of LK 3.43 cm and RK 3.35 cm) at GA 34w5d corresponded accurately while conventional biometry underestimated by 2–3 week.