Fetus

In human development, the term fetus refers to the developing unborn baby from roughly the ninth week after fertilization (about 11 weeks of gestational age) until birth. The fetal period follows the embryonic stage and is characterized primarily by growth and maturation of tissues and organs that formed earlier.

During this period, the fetus develops inside the uterus, enclosed by the amniotic sac and filled with amniotic fluid. It is connected to the placenta by the umbilical cord. The placenta enables the transfer of oxygen and nutrients from the mother's blood to the fetus and the removal of waste products from fetal blood.

Over the second and third trimesters, most major organ systems continue to mature. The brain and lungs undergo significant development, and the fetus grows in size and weight. Body tissues differentiate, and stores of fat increase in preparation for birth. Movements caused by muscular development become noticeable to the mother in the second trimester. Fetal sex can often be determined by ultrasound around mid-pregnancy, and the fetal heartbeat is detectable by ultrasound or Doppler devices early in pregnancy.

Viability, or the ability to survive outside the uterus with medical support, is generally around 24 weeks of gestation, though outcomes improve with advancing gestational age and access to intensive care. Prenatal care typically includes routine screening, ultrasound monitoring, and, when indicated, interventions for fetal growth or health concerns. The term fetus is used in both medical and general contexts to describe this stage of prenatal development.