Regular contractions are characterized by the following three aspects: I. Rhythmicity. A normal contraction is an involuntary, rhythmic paroxysmal contraction of the uterine body. Each contraction is weak and strong, maintained for a certain period of time, and then strong and weak until it disappears into the interval. During the interval, the uterine muscles relax and the contractions recur until the end of the whole process of labor. Second, symmetry and polarity. Normal contractions start from both corners of the uterus, move to the center of the uterine fundus, and then expand to the lower section, symmetrical left and right, called the symmetry of uterine contractions. Contractions to the bottom of the uterus is the strongest, the most persistent, downward gradually weakened called polarity. Third, the role of contraction. The smooth muscle of the body of the uterus is the active contraction part, whenever the contraction, the muscle fibers of the body of the uterus shorten and widen, the muscle fibers relax during the interval, but can not fully return to the original length. After repeated contractions, the muscle fibers become shorter and shorter, which is called “shrinkage”. With repeated contractions of the uterus, the muscle fibers become thicker and shorter, and the upper segment of the uterus becomes thicker and thicker, while the lower segment is stretched and expanded and becomes longer and thinner. The junction of the upper and lower segments forms a ring-like groove due to the different thickness of the muscle layer, which is called the physiological contraction ring.