The different stages of human development. The fetal stage of human development begins around 8 weeks after fertilization — when the embryo has developed its basic form — and lasts until birth. It’s a transformation unlike any other: from a 1-inch-long, bean-shaped body to a fully formed newborn, 20 inches (50 cm) long and weighing around 7.5 pounds (3.4 kg). By the end of the first trimester, most major organs are in place. The heart has been beating since about week 6, but now it’s pumping blo

Human embryonic development. | Hashem Al-Ghaili