According to the search results, the last major organ to finish developing in a fetus is the lungs. The lungs produce a chemical that affects the hormones in the mothers body when they are fully mature, and this chemical may be one of the causes of labor starting. However, it is important to note that almost all organs are completely formed by about 10 weeks after fertilization, which equals 12 weeks of pregnancy, except for the brain and spinal cord, which continue to form and develop throughout pregnancy.