Embryo development begins with fertilization, where a sperm cell from a male merges with an egg cell from a female. This union forms a single cell called a zygote, which contains genetic material from both parents. The zygote then undergoes rapid cell division, transforming into a multi-cellular structure as it travels down the fallopian tube toward the uterus. Once in the uterus, the developing embryo implants itself into the uterine lining. Over the next several weeks, it goes through various stages, forming essential structures like the heart, brain, and limbs. By the end of the first trimester, the embryo is recognizable as a developing fetus.