Contractions are characterized by paroxysmal pain with short intervals of 5-6 minutes, gradually increasing in duration, lasting 30 seconds to 1 minute, causing dilatation of the uterine orifice or disappearance or shortening of the cervical canal, which cannot be suppressed by sedative drugs. Contractions have the following characteristics: 1, rhythmic: rhythmic to the fetal blood perfusion is favorable. 2, symmetry: contractions start from both sides of the uterine horn, to the bottom of the uterus in the midline, symmetrical. 3, polarity: the bottom of the uterus contraction is the longest and most persistent, to the cervical canal in order to weaken. 4, contraction: uterine contraction when the myofibers shorten to become wider; myofibers in the interval can not be fully restored to the original length, resulting in the gradual reduction of the uterine cavity. 5, contraction: uterine contraction is a very important part of the uterus. The volume of the uterine cavity is gradually narrowed.