Nonmalignant growths within the wall of the uterus that may expand during pregnancy. Most common in women over 35 years of age. Occasionally, these cause problems, increasing slightly the risk of ectopic pregnancy, miscarriage, placenta previa, abruptio placenta, premature labor, premature rupture of the membranes, stalled labor, fetal malformation, and breech and other more difficult to deliver fetal positions.