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Labor is defined as regular contractions, which result in dilation (opening) and effacement (thinning) of the cervix. Labor is usually divided into four stages:

Stage 1 starts with the onset of regular contractions and ends when the cervix is completely effaced and dilated. Voluntary pushing with the abdominal muscles is not helpful and may result in tearing the cervix.

Stage 2 starts with complete dilation of the cervix and ends when the baby is delivered. It involves the movement of the baby through the birth canal and is completed with the delivery of the baby. Birth is aided by voluntary bearing down, using the abdominal muscles.

LABOR BIRTH GRAPH
(Friedman labor curve)

Stage 3 refers to delivery of the placenta. The uterus will deliver the placenta on its own; no bearing down is required.

Stage 4 refers to the first few hours after the delivery of the placenta, when the uterus shrinks in size and you become acquainted with your baby.

Although all labors have these same four stages, each labor is unique. Labors differ from one woman to the next and even from one labor to the next in the same woman. Some of the factors that affect the course of labor include general states of health and nutrition, emotional behavior, preparation for childbirth, size and shape of the pelvis, size and presentation of the baby, and medical interventions.

The cause of the onset of labor is not completely known. The following factors are thought to contribute to the start of labor:

1. The uterus reaches a critical point of distention
2. The placenta ages, and levels of the hormone progesterone drop, causing the uterus to become irritable
3. Hormones from the baby stimulate uterine contractions
4. The mother’s pituitary gland releases oxytocin, a hormone that causes the uterus to contract
5. Hormones called prostaglandins are released from the wall of the uterus, stimulating contractions of the uterine muscle
6. Stretching of the cervix sets up reflexes in the nervous system that stimulate more contractions

There is no conscious action on the part of the mother that can start or stop contractions. However, anxiety, fatigue, physical activity, and sexual stimulation may affect the contractions. Rides over bumpy roads do not seem to start labor.

Throughout the last two-thirds of your pregnancy, the uterus has mild, intermittent contractions without pattern. These are referred to as Braxton-Hicks contractions. The contractions are exercises that condition the uterus for labor, and they are felt as a tightening in the lower abdomen that lasts from a few seconds to a few minutes. They have no rhythmical pattern. You may notice that the uterus feels hard during these contractions.

The Braxton-Hicks contractions gradually become stronger and occur more frequently as you near the end of your pregnancy. Often these contractions cause very gradual, intermittent softening, thinning, and opening of the cervix. Since these changes are not steadily progressive, they are not considered labor.

The contractions of labor are made up of three parts: the build-up, the peak, and the tapering off. Between contractions, there are periods of relaxation so that the body can rest and the uterus will be re-oxygenated.

The frequency of a contraction is timed from the beginning of one contraction until the beginning of the next. Duration is timed from the beginning of a contraction to the end of the same contraction.