In human beings, after one month of pregnancy, the embryo’s heart is formed. By the end of the second month of pregnancy, the fetus develops limbs and digits. By the end of 12 weeks (first trimester), most of the major
organ systems are formed. The first movements of the fetus and appearance of hair on the head are usually observed during the fifth month. By the end of 24 weeks (second trimester), the body is covered with fine hair, eye-lids separate, and eyelashes are formed. By the end of nine months of pregnancy, the fetus is fully developed and is ready for delivery.