(1) Expulsion from the uterus of an embryo or fetus before viability (20 weeks' gestation [18 weeks after fertilization] or fetal weight less than 500 g). A distinction made between abortion and premature birth is that premature infants are those born after the stage of viability but before 37 weeks' gestation. Abortion may be either spontaneous (occurring from natural causes) or induced (artificially or therapeutically). (2) The arrest of any action or process before its normal completion.