The mechanism of labor is in several stages. In pre-labor, the uterus begins to contract, but the contractions are more gentle. The pregnant woman can usually go about her day. In the first active phase of labor, the uterus contracts more rhythmically and frequently, usually requiring undivided attention. These contractions coincide with the cervix dilating to about 10 cm to allow for the baby to pass through. In the second active phase of labor, the doctor or midwife will ask that the woman push each time a contraction occurs. This assists the uterus contractions in pushing the baby out. Finally, in the third phase, the placenta is expelled.