The duration of human pregnancy, counting from the first day of the last menstrual period, is ˜280 days, or 40 weeks. The expected date of delivery can be estimated by counting back 3 months from the first day of the last menstrual period and adding 7 days (Nägele’s rule). Approximately 40% of women begin labor within 5 days of the estimated date of confinement (EDC) and nearly two-thirds do so within 10 days.

In most species, including the human, the uterus remains relatively quiescent for most of the pregnancy. The contractions that occur are poorly synchronized and of low amplitude and develop only small increases in uterine pressure. This pattern contrasts with the high-frequency, high-amplitude contractions of the myometrium that occur at the time of labor. Thus, the uterine changes can be divided into distinct phases. For most of pregnancy, the myometrium is relatively quiescent, corresponding to phase 0 of parturition. In late gestation, the myometrium undergoes a transition from a state of quiescence to one of activation, which physiologically corresponds to phase 1 of parturition. Clinically, the first stage of labor begins with the first uterine contraction and ends with complete dilatation of the cervix. The second stage begins with complete dilatation of the cervix and ends with the birth of the baby. During this phase, the activated uterus responds to stimulation by a variety of uterotonins. Involution of the uterus postpartum is termed phase 3. This stage begins with delivery of the baby and ends with the expulsion of the placenta. The mean duration of labor in primigravid women is ˜14 hours; the duration of labor in multiparous women is ˜6 hours shorter. In this sequence, the term “initiation of parturition” most closely corresponds to the transition from phase 0 to phase 1.

During the first stage of labor the cervix undergoes progressive effacement in which the cervical canal shortens from a structure ˜2 cm in length to one in which the edges of the orifice are paper thin, and dilatation in which the orifice to the uterus (internal cervical os) enlarges from a few millimeters in diameter to an aperture large enough to permit passage of the baby (10 cm or complete dilatation). At labor, the uterus gradually differentiates into an upper, actively contracting portion, which thickens as labor advances, and a lower portion composed of the lower uterine segment and cervix, which thins remarkably. Spontaneous
rupture of the membranes usually occurs during labor, generally manifested by a sudden gush of clear or slightly turbid fluid. The uterus changes in shape during each contraction and as labor progresses towards the final expulsion of the infant.