Usually, miscarriages happen when an embryo or fetus is not developing normally. Often due to genetics or chance, the developing fetus will have too few or too many chromosomes, information in cells that guide growth and allow a pregnancy to progress. Being older can increase the risk of chromosome abnormalities, and one-third of pregnancies in people ages 40 and older end in miscarriage. Sometimes, abnormalities in the cervix or uterus can lead to miscarriage, too. But miscarriages very rarely happen because of anything the pregnant individual has done. Work, exercise, stress, arguments, sex, and morning sickness won't cause a person to miscarry.