Human development begins when a sperm enters and fuses with an egg, forming a single cell called a zygote. This cell divides repeatedly while traveling toward the uterus. As divisions continue, it forms a blastocyst that attaches to the uterine wall. After implantation, the embryo develops rapidly, laying down the basic structure of all major organs. By the end of the eighth week, the embryo becomes a fetus. Over the next months, the fetus grows, organs mature, bones strengthen, and systems beco