Final answer:
True labor progresses in a positive feedback loop with uterine contractions stretching the cervix, causing it to dilate and efface. Cervical dilation triggers more powerful contractions through the release of oxytocin. Contractions become more frequent and intense as labor progresses.
Step-by-step explanation:
True labor progresses in a positive feedback loop in which uterine contractions stretch the cervix, causing it to dilate and efface, or become thinner. Cervical stretching induces reflexive uterine contractions that dilate and efface the cervix further. In addition, cervical dilation boosts oxytocin secretion from the pituitary, which in turn triggers more powerful uterine contractions. When labor begins, uterine contractions may occur only every 3-30 minutes and last only 20-40 seconds; however, by the end of this stage, contractions may occur as frequently as every 1.5-2 minutes and last for a full minute.