Human embryo

A human embryo is the earliest stage of human development, beginning when a sperm fertilizes an egg and continuing through roughly the first eight weeks of pregnancy. During this remarkable period, a single cell rapidly divides and organizes itself into the basic structures of a human body, including the heart, brain, and limbs, transforming from microscopic to about the size of a grape. Scientists study embryos to understand how life develops and to advance medical treatments, though research involving human embryos raises important ethical questions that societies continue to debate.