fetuses

In human biology, a fetus is the developing mammal from roughly the end of the embryonic period until birth. The embryonic period ends at about eight weeks after fertilization, or roughly ten weeks after the last menstrual period. The term “fetus” is used in medical contexts for humans and many vertebrates, with “foetus” common in some regional spellings.

During gestation, the fetus undergoes rapid growth and organ maturation. The first trimester sees the formation of major organ precursors; by the second trimester most organ systems are present but immature. In the third trimester, growth accelerates and the lungs, brain, and other systems continue to mature in preparation for breathing outside the womb. Prenatal ultrasound and screening assess development and potential concerns throughout pregnancy.

The fetus is connected to the mother via the placenta and the umbilical cord, which supply oxygen and nutrients and remove waste. Fetal circulation includes structures such as the ductus arteriosus and foramen ovale that redirect blood flow before birth. The amniotic sac and fetal movements support growth, fluid balance, and development of muscles and bones.

Medical care during pregnancy often includes regular prenatal visits, ultrasound imaging, and fetal monitoring to track growth and well-being. Viability—the ability for a fetus to survive outside the womb—improves with gestational age; with current medical care, many fetuses born around 23–25 weeks’ gestation survive, though outcomes vary.

Terminology varies by region: “fetuses” is the American English plural, while “foetuses” is common in British English. The topic intersects medicine, ethics, and law, reflecting ongoing discussions about fetal development and personhood.