Gestational age assurance is one of the best strides in the administration of the timing of pregnancy and subsequent diminishing in perinatal mortality due to preterm conveyances. There is no single boundary to assess the gestational age. The utilization of ultrasound (US) biometric routine boundaries, including biparietal width (BPD), head outline (HC), stomach circuit, femoral length (FL), and fetal weight, is obligatory at all pregnancies beginning from 20 weeks. However, at late incubation , these boundaries are not exact. This examination expected to assess the precision of fetal kidney length by ultrasonography in assessing gestational age in third trimester. Techniques: This investigation is cross sectional examination that was done at Benha University clinic during the period from March 2019 to March 2020.Routine U/S assessment and kidney assessment. Estimations of all fetal biometric boundaries were acquired. Results: When contrasting the determined GA and LMP with various boundaries, the mean of LMP GA was (36.5±2.6), BPD GA was (36.0±2.72), HC GA was (35.8±2.70), AC GA was (35.5±2.9), FL GA was (36.5±2.73).There were measurably critical positive connection between's mean fetal kidney length and (biparietal breadth (BPD), BPD GA, head outline (HC), HC GA, stomach boundary (AC), AC GA, femoral length (FL) and FL GA. The current investigation determined a condition for assessment of the gestational age from the fetal kidney length (FKL) with SE ±2.4 days. End Fetal kidney length increments with increment in fetal gestational age (FGA) and shows incredible intra and bury class connection coefficient which recommends great understanding and reproducibility of estimations.