The term embryo applies to the product of conception up to the formation of organs, i.e., until the end of the eighth week of gestation. From that point until the end of pregnancy, the product of conception is called a fetus.

The embryo evolves into a fetus at the end of the second month, after the eighth week, when it measures c. 30 millimeters and weighs c. 4 grams. Until the 180th day of endo-uterine life, the fetus is not vital. The fetus is at term after 38 weeks (260 days) of gestation; a birth before the 38th week is defined as premature. At the term of pregnancy, which normally lasts 280 days from the start of the last menstruation, the fetus is 48-52 centimeters long and weighs 3-3.5 kilograms.