What do we call an unborn infant from the end of the third month of pregnancy until birth?

The term used to describe an unborn infant from the end of the third month of pregnancy until birth is "fetus." This period, also known as the fetal stage, marks significant developments in growth and organ maturation. During this time frame, the fetus undergoes rapid growth, starting to develop features recognizable as human, such as limbs and facial structures.

The term "embryo" refers to the early stages of development from conception to the end of the eighth week of pregnancy, so it does not apply to later stages. "Neonate" is used to describe a newborn baby within the first 28 days after birth, while "infant" generally refers to a young child from birth up to 12 months old. Therefore, these terms pertain to different stages in the life cycle, making "fetus" the appropriate term for the specified period of development.