Normal labor refers to the presence of regular uterine contractions that cause progressive dilation and effacement of the cervix and fetal descent. It is viewed in terms of its phases: latent, active, and decelerative; and its stages: first, second, and third.
The latent phase of labor will be reviewed here. Other aspects of labor and labor abnormalities are discussed separately:
Latent phase — The initial phase of labor is termed the latent phase. It begins at the point at which the woman perceives regular uterine contractions. These contractions gradually soften, efface, and begin to dilate the cervix. This simple definition belies a complex process not completely understood by modern science. Women have irregular contractile activity beginning in the midtrimester, with gradual alterations in the size, shape, and consistency of the cervix occurring over the remaining months of the pregnancy [1,2]. This makes accurate assessment of the onset of latent phase difficult since determining exactly when a patient has entered latent phase is primarily based upon the woman's history and the judgment of the attending provider.
The end of the latent phase occurs when the active phase begins. Precise determination of the time that latent phase ends is particularly problematic in an individual woman. Zhang and colleagues  noted that parturients have diverse speeds of cervical dilation in the latent versus active phases and enter the active phase at various times (as measured by cervical dilation). Although some woman may enter the active phase at less than 6 cm, beyond a dilation of 6 cm, rates of cervical dilation are more rapid in both nulliparas and multiparas. Thus, it is clinically practical to consider all women to be in active labor once they have reached a dilation of 6 cm or more. (See "Overview of normal labor and protraction and arrest disorders", section on 'Characterization of normal progress'.)