Background: Accuracy in estimation of gestational age (GA) has become more demanding as technology used in ultrasonography has become more advanced. Patient's expectations have also increased and they expect exact estimations to plan their pregnancies. Mean fetal kidney length (MFKL) alone or in combination with other biometric indices can be used for estimating GA more precisely.
Objective: To measure the length of fetal kidney in normal singleton pregnancies in late second and early third trimesters, its correlation with GA derived from the first day of patient known last menstrual period (LMP) and comparison with GA derived from other conventional biometric methods.
Patients and methods: This study was a prospective observational study, conducted at the department of Obstetrics & Gynecology, Obstetrics & Gynecology Hospital, Assiut, 500 pregnant females were enrolled in this study from March 2018 to March 2020. Those women underwent measurement of the right and left kidneys and mean was taken. Comparison between gestational ages was determined by LMP. In addition, mean fetal kidney length as well as between mean fetal kidneys length and other fetal biometric indices was done.
Results: Baseline data of enrolled women (n= 500) mean age of those women was 26.31 ± 4.87 years. Median parity and abortion was 2 with range of both was between 0-6 and 0-8 times, respectively. Mean gestational age was 30.29 ± 3.34 weeks while mean body mass index was 19.84 ± 1.20 kg/m2. Average fetal kidney length was 29.88 ± 3.33 mm. Gestational ages based on different fetal biometric indices.  Age by date had strong positive significant correlation with different fetal indices but the strongest correlation was observed with mean fetal kidney length (r= 0.96, P< 0.001). Correlation between gestational age by date and expected age by MFKL based on regression equation.  That there was significant strong positive correlation between gestational age and expected age by MFKL (r= 0.97; P< 0.001).
Conclusion: Mean fetal kidney length can be used as an accurate new parameter for estimation of gestational age in combination with BPD, HC, AC & FL especially in late second and early third trimester (beginning from 24th week – 34th week). Using regression equation: Gestational age by date (weeks) = 1.56+ 0.97 (MKL in mm).