Ultrasound measurement of fetal kidney length in normal pregnancy and correlation with gestational age

Belated booking for antenatal care remains a common practice among pregnant women in developing countries. Many of the existing conventional ultrasonographic indices used for assessing gestational age (GA) become increasingly unreliable beyond the 2^nd trimester.

A prospective cross-sectional study of booked healthy pregnant women with regular menstrual cycles and certain last menstrual period (LMP) were examined using ultrasound over a 6-month period. Fetal kidney length (FKL) was measured, beginning from 20 to 41 weeks of gestation.

A total of 400 pregnant women were examined. The mean left FKL was slightly but significantly higher than the mean right FKL across the various weeks of gestation (P < 0.05). The mean combined FKL (MKL) was 32.18 ± 5.96 mm, with a reference range of 20.87 ± 0.75 mm to 41.41 ± 0.07 mm from 20 to 41 weeks of gestation. There was strongly positive significant correlation between MKL in millimeters and GA by the LMP in weeks (r = 0.997, P = 0.000). The standard error of prediction (SE) of GA was least for MKL (±7.17 days), followed by femur length (FL) (±8.38 days), head circumference (HC) (±9.02 days), abdominal circumference (AC) (±9.14 days), and biparietal diameter (BPD) (±10.36 days). The best multiple regression model for predicting GA included the five variables such as MKL, FL, BPD, HC, and AC, with a SE of prediction of (±6.31 days).

FKL can serve as an additional reliable parameter for accurate dating of GA during obstetric scan.