The strong, rhythmic tightening of the muscles of the uterus, increasing in intensity, frequency, and duration, that accompanies labor. Uterine contractions cause the cervix to dilate (widen) so the baby can pass into the vagina. At first, the contractions may be mild, irregular, and infrequent. Early contractions can feel like gas pains, menstrual cramps, or a backache. As labor progresses, the contractions become stronger and more regular. Relaxation between contractions helps reduce pain and conserve energy for the next contraction. Preceding labor, particularly in the last month or two of pregnancy, sporadic, mild contractions called Braxton Hicks contractions may occur. They are not associated with the progressive opening of the cervix or the pain seen with labor contractions.