Labor is defined as the physiological process by which the pregnant uterus is emptied of its contents - namely, the fetus and the placenta - through the birth canal. It is classically divided into three stages.
The first stage consists of the dilatation of the cervix, the second stage refers to the expulsion of the baby through the fully dilated cervix, whereas the third stage refers to the delivery of the placenta and the retraction of the uterus to close off the open maternal sinuses which fed the placenta.
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Phases of the First Stage of Labor
The first stage of labor includes the dilatation of the cervix from zero to 10 centimeters, at which the cervix is said to be fully dilated. It is further subdivided into three distinct phases.
Initially there is the latent phase which is usually long and not painful. Extending over hours, days or weeks, it consists of the effacement and dilatation of the cervix. Effacement refers to the thinning out of the cervix from a firm tubular structure which holds the fetus inside the uterus to a thin, circular rim surrounding the uterine opening through which the fetus will enter the birth canal. This then dilates progressively and slowly to 3 centimeters, at which stage the next phase begins as below. Mild contractions may occur, but may not be regular, or noticeable.
The active phase of labor is marked by the onset of frequent and painful contractions occurring about every three or four minutes, lasting for about 30 to 60 seconds. This results in the continuing dilatation of the cervix from 3 to 7 or 8 centimeters.
The transition phase of labor is the final part of the first phase, when the cervix dilates to 10 centimeters. The contractions are very intense and painful in most women in this phase, and result in the baby moving down the birth canal and being born.
Signs of Latent Phase Labor
Some signs of the onset of the latent phase of the first stage of labor include abdominal cramps like those associated with menstruation, diarrhea without any other evidence of intestinal infection and small amount of spotting (or even bleeding) that is often associated with a mucous discharge due to the expulsion of the loosened mucus plug with progressive slow cervical dilatation (which is called the ‘show’).
Low dull backache thought to be associated with cervical dilatation often precedes established labor, though for many women it is more of a feeling of pressure low down in the pelvis. These symptoms are notable in not being amenable to relief by ordinary measures.
A clear watery gush or trickle may occur if the membranes break releasing amniotic fluid, which usually occurs later in labor rather than at the onset. This symptom should always be reported to the healthcare provider. In a majority of cases labor sets in within 24 hours of membrane rupture. If the fluid release occurs before the onset of labor, it indicates premature rupture of membranes and medical help should be sought to ensure that appropriate measures are taken to avoid fetal infection. In complicated cases the amniotic fluid may be bloody, discolored, or smelly, and this indicates the need for immediate medical attention.
Lightening refers to the descent of the fetal presenting part into the pelvis, making more room for the mother’s lungs under the rib cage and so producing a sense of relief. This usually happens in the last month for primigravidas (i.e. women pregnant for the first time) but may occur only during active labor after the first childbirth. In addition, frequent urination may be noticed because of the pressure of the fetus on the bladder after lightening occurs.
Strange impulses to compulsively complete preparations for the baby’s birth, such as a thorough housecleaning, finishing the furnishing of the nursery or the baby’s layette, or stocking the refrigerator with several meals, are collectively called ‘nesting’ and may be a sign of impending labor.
Signs of True Labor
True or established labor refers to the onset of the active phase of the first stage of labor. It is characterized by painful uterine contractions which are regularly spaced but start to come sooner and sooner, eventually occurring every few minutes, and which increase progressively in intensity and length.
The pain is strong enough to cause real discomfort and is not easy to tolerate. These are different from the Braxton-Hicks contractions which are short and painless, as well as irregular and unpredictable contractions occurring throughout the latter half of pregnancy, and are not related to cervical dilatation. Also, these often disappear with a change of activity or of posture, unlike true labor pains which do not respond to any form of palliative activity.
Reviewed by: Dr Tomislav Meštrović, MD, PhD