Miscarriage is one type of pregnancy loss within the larger category of perinatal loss. Perinatal loss is the death of a fetus or infant during pregnancy or around the time of birth. Medically, the most up-to-date definition of miscarriage, also termed spontaneous abortion, understands it to be the involuntary expulsion of a fetus prior to 20 weeks of gestation, or of a fetus weighing less than 500 grams. Weight is a factor because calculation of gestation is subject to inaccuracy. Both gestation and weight relate to fetal viability, most commonly taken to be age at which a fetus is able to survive outside of the womb.