Labor is divided into two phases, the latent phase and the active phase. The latent phase begins with the onset of regular uterine contractions. When a woman is pregnant for the first time, this phase can last up to 18 hours. When a woman has already had a baby, the latent phase is usually complete within 12 hours. Sometimes, cervical ripening can occur prior to the onset of labor and in these cases, the latent phase is usually shorter.
Active labor is the phase in which there is more rapid cervical dilatation.
Active labor usually begins at about 4cm of dilatation. The average rate of dilatation is 1.2cm per hour for a first pregnancy and 1.5cm per hour in subsequent pregnancies. The first stage of active labor ends with complete dilatation of the cervix. The second stage involves the descent of the fetal head through the pelvis and delivery of the baby. On average, this lasts 1-2 hours for a first baby and less than an hour for others.