Kidney development begins in the early stages of embryonic growth. The kidneys form from three sets of structures: the pronephros, mesonephros, and metanephros. The metanephros is the final and functional kidney that develops around the fifth week of gestation and continues to mature throughout pregnancy. As the embryo grows, the metanephros undergoes significant changes, including the formation of nephrons, the functional units of the kidney. By the end of the first trimester, the kidneys are capable of producing urine, which contributes to the amniotic fluid. This process is crucial for maintaining a healthy environment for the developing fetus.