Fetus

A developing mammal in the stage after the embryonic period and before birth or hatching. In humans, this stage begins about eight weeks after fertilization.

Gestational age: The duration of pregnancy, usually measured from the first day of the last menstrual period. It provides the standard timeline for describing fetal development in human pregnancy.
Embryo: An organism in its early developmental stage, before the fetal stage in mammals that use this distinction. The transition from embryo to fetus marks the end of the early developmental period.
Prenatal diagnosis: Medical testing used to assess fetal health or identify genetic and structural conditions before birth. It can detect some fetal conditions during pregnancy and inform clinical care.
Premature birth: Birth before 37 completed weeks of human pregnancy. A prematurely born fetus must continue development outside the uterus.
Placenta: A temporary organ that exchanges gases, nutrients, and waste between a pregnant mammal and its developing young. It supplies the fetus and removes waste while keeping maternal and fetal blood largely separate.
Fertilization: The fusion of sperm and egg cells to form a zygote. It begins the developmental sequence that eventually reaches the fetal stage.
Ultrasonography: Medical imaging that uses high-frequency sound waves to produce images of structures inside the body. Ultrasound is widely used to assess fetal growth, anatomy, and position.
Stillbirth: The loss of a fetus before or during birth after a threshold defined by local medical or legal standards. It describes fetal death in pregnancy or around delivery, not a developmental stage.
Umbilical cord: A flexible structure connecting a fetus to the placenta through blood vessels. Its vessels carry oxygen and nutrients toward the fetus and waste products away.
Zygote: The single cell formed at fertilization, before it begins dividing. It is the earliest stage in the same continuous development that later includes a fetus.

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