Embryogenesis of the kidneys includes three stages: Pronefros - is located behind the peritoneum on the dorsalateral surface of the body mesonephros - formation of tubules and duct ducts takes place; metonephros (permanent kidney) - direct development of the kidney, i.e. its "maturation".
The kidneys in the embryo begin functioning on the 9th week of intrauterine existence. The formed urine is excreted into the amniotic fluid. Since the placenta is the main excretory during the prenatal period, children with renal agenesis are born alive and die after cutting the umbilical cord due to renal failure.
In newborns, compared with adults, the lumen of the tubules is almost half narrower, they are much shorter, the size of the glomeruli is reduced. This leads to a decrease in reabsorption and filtration. Ureters have many bends, their diameter is 0.3-0.4 cm. The muscle and elastic layers are insufficiently developed in the bladder, its capacity is 30 ml. Urethra in girls is wider than in boys, its length is 1-2 cm in average, in boys - 5-6 cm. This should be taken into account when using catheterization and cystoscopy.