A fetus is the scientific name for a developing baby inside its mother's womb, specifically from about the ninth week of pregnancy until birth. Before the ninth week, scientists call it an embryo.

During the fetal stage, the developing baby grows rapidly. Its organs continue maturing, bones strengthen, and it develops the ability to move, hear sounds, and even respond to light. By the time a baby is born, it has spent about nine months developing before birth, growing from something smaller than a grape into a fully formed infant ready for the world.

Doctors use the word fetal to describe things related to this stage of development. For example, a fetal heartbeat is the sound of the developing baby's heart, which doctors can detect with special equipment. Scientists who study how babies develop before birth work in fetal medicine.

You'll encounter this word in science classes when learning about human development, in discussions about pregnancy and childbirth, and in medical contexts. While some people prefer to say “unborn baby” in everyday conversation, fetus is the precise scientific term that doctors, nurses, and biologists use to describe this specific stage of development before birth.