![]() The first stage of labor can be considered the cervical stage, the second stage the pelvic stage (reflecting the descent of the fetus through the pelvis), and the third stage the placental stage. The third stage begins with the birth of the infant and ends with the delivery of the placenta. The second stage of labor begins with the complete dilation of the cervix and ends with the birth of the infant. ![]() Preterm fetuses require less than 10 cm of cervical dilation. At term gestation, 10 cm approximates complete cervical dilation. Complete cervical dilation is the dilation necessary to allow movement of the fetus from the uterus into the vagina. The first stage begins with the maternal perception of regular, painful uterine contractions and ends with the complete dilation of the cervix. Preterm and post-term deliveries both constitute important obstetric problems and when more is understood about the mechanism of the onset of labor, new approaches to preventing the preterm and post-term onset of parturition may evolve.īy convention, labor is divided into three stages. 1, 2 As more is learned, perhaps a unifying concept of the onset of mammalian labor will emerge. The laboring human uterus does manifest increases in prostaglandin production, oxytocin receptors, and myometrial gap junction formation. In women, progesterone concentrations do not decline before the onset of labor and no surge in fetal cortisol secretion occurs. In sheep, the fetus apparently triggers parturition through a surge in fetal cortisol production. In other mammalian species, a decrease in serum progesterone concentration in association with an increase in estrogen concentration is followed by increases in prostaglandin production, oxytocin receptors, and myometrial gap junction formation. The cause of the onset of labor in women-either term or preterm-remains unknown.
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