Labor: Childbirth, the process of delivering a baby and the placenta, membranes, and umbilical cord from the uterus to the vagina to the outside world. During the first stage of labor (which is called dilation), the cervix dilates fully to a diameter of about 10 cm (2 inches). The first stage of labor is divided into two phases: the latent phase and the active phase. In the latent phase, contractions become progressively more coordinated and the cervix dilates to 4 cm (approximately 1.5 inches). The latent phase averages about 8 hours for a nullipara (a woman having her first baby) and 5 hours for a multipara (a woman having a subsequent baby). In the active phase, the cervix becomes fully dilated and the presenting part of the baby descends into the midpelvis. The active phase averages about 5 hours for a nullipara and 2 hours for a multipara. In the second stage (which is called expulsion), the baby moves out through the cervix and vagina to be born. Expulsion generally lasts 2 hours for a nullipara and l hour for a multipara. The third stage of labor begins with the delivery of the baby and ends when the placenta and membranes are expelled. Also known as parturition and childbirth.