Biologic basis of term and preterm labor.

Normal parturition in sheep and nonhuman primates appears to be initiated by signals from the fetus. These signals involve the fetal hypothalamo-hypophyseal-adrenal axis and are closely integrated with the control of maturation of the fetal organs such as the lungs, gut, and kidneys that are indispensable for extrauterine life. The integrated set of changes in fetal and maternal myometrial, decidual, and cervical tissues are gradual and occur over several days. When the myometrium and cervix have been appropriately prepared, endocrine and paracrine factors in the fetal membranes, decidua, and the myometrium bring about a change in the pattern of myometrial activity from contractures to contractions. This switch occurs at night, and recurs and augments over several nights until eventually cervical dilation occurs to allow the fetus to be born.