A fetus is an unborn, developing mammal. The fetal stage comes after the embryo stage, after the eighth week of gestation or the 10th week of pregnancy, and lasts until birth. A normally developing embryo will have begun to form major identifying features at the end of the eighth week, such as its nose and mouth. Its organs will also have begun to develop and will continue to do so until it is born.
As an embryo develops into a fetus in a pregnant woman, it grows arms and legs and begins to develop fingers and toes. It also begins to grow ears and eyelids. The embryo also begins to form lungs in the eighth week of pregnancy, and its brain continues developing. At this stage in the pregnancy, the embryo is still very small, at less than 0.75 of an inch long.
In the beginning of the fetal period of a pregnancy, starting at week nine of gestation, the baby closes its eyelids. The eyes remain closed until the pregnancy is nearly over. It genitals have begun to develop, and it is beginning to produce red blood cells in its liver. During weeks nine to 12, the face will become fully formed, and it will be able to clench its hand into a fist.
Throughout the rest of the pregnancy, the baby will continue to grow and develop. It will grow hair, eyelashes, and nails. The body will begin to store fat, and its respiratory system will develop and mature over the course of the pregnancy.
Although most fetuses are able to move by the eighth week, most mothers are unable to feel the movement until around the 20th week, or halfway through a human pregnancy. During the 20th week, the fetal heartbeat becomes audible through a stethoscope outside of the uterus.
The definition of a fetus can be a source for debate. Some people believe that a human child is not fully a human child until born and that abortion is acceptable because the fetus is not yet a baby. Other people argue that life begins once the egg is fertilized, so even an embryo qualifies as a living being with rights to life. Some debates focus on whether a fetus can feel pain inside the womb and whether it is acceptable to show a pregnant woman ultrasound images as she makes a decision to abort.