Labor is defined as contractions that change the uterine cervix, which results eventually in the delivery of the fetus. Labor is considered one of the most intense experiences of pain widely encountered. The cause of pain might be explained by hypoxia of the contracted uterine muscle (as in myocardial infarction), stretching of the cervix during dilation, or compression of nerves in the cervix.
Labor starts with mild contractions or tightening of the muscles of the uterus. Regular uterine contractions cause softening and effacement of the cervix (i.e., thinning of the cervix) and dilation of the cervix. The degree of cervical effacement is expressed in terms of the length of an unaffected cervix. Before labor, the uterine cervix is about 4 centimeters long and tightly closed. Thus, when the length is reduced by one half, it is defined as 50% effacement, and thinning of the cervix to the thickness of a piece of paper means 100% effacement. The dilation is ascertained by estimating the diameter of the cervical opening. The final stage of dilation, enabling the passage of the baby into the birth canal, is of 10 centimeters opening (i.e., full dilation). Uterine contractions that do not cause cervical dilation are called Braxton-Hicks contractions, which may be observed at any time during pregnancy, but are more common near the end of pregnancy.
Two phases of cervical dilation are the latent phase and the active phase. Latent phase is characterized by contractions leading to cervical softening and the beginning of dilation of the cervix. Active labor means that contractions are regular (three contractions in 10 minutes) and strong, usually beginning at cervical dilation of 3 to 4 cm, resulting in progressive dilation. Much energy is expended during this process, which expresses the term labor. A clinical sign of the impending onset of active labor is the discharge of a mucus plug with a few drops of blood that occluded the cervical canal during pregnancy (i.e., “bloody show”).
Active labor is divided into three stages. The first stage is defined as regular contractions leading to effacement and progressive dilation of the cervix, ending when the cervix is fully dilated to 10 centimeters. The second stage is the expulsion of the fetus, starting at 10 centimeters. When the cervix is completely open, the patient feels a strong urge to push, which drives the baby down the birth canal. This stage of labor ends as the baby is born. The third stage, involving several contractions, is the delivery of the placenta and fetal membranes, starting immediately after the delivery of the baby. Disproportion between the contracted uterus and the implantation area of the placenta enables its complete detachment from the uterine wall.
Spontaneous rupture of the membranes, characterized by a sudden gush or trickle of amniotic fluid from the vagina, typically occurs during the active phase of labor. This is a signal that the amniotic membranes have broken. However, rupture of the membranes before the onset of labor is referred to as premature rupture of membranes.
Friedman had first described the normal progress of labor in different curves for nulliparous (women at their first delivery) and multiparous women (second delivery and more). According to Friedman, the normal progress during the first stage of labor is cervical dilation rate of about 1.2 cm/hour for nulliparas and 1.5/cm hour for multiparas. The length of the second stage of labor was limited to 2 hours in nulliparous women (or 3 hours if epidural analgesia was applied) and 1 hour in multiparous women (or 2 hours if epidural analgesia was applied). Deviation from these curves has been defined as failure to progress in labor (labor dystocia, or abnormal labor). When time breaches in normal labor limits occur, interventions such as augmentation of labor or even instrumental or operative delivery might be considered.
References:
- American College of Obstetricians and Gynecologists, http://www.acog.org
- Friedman, A. (1978). Evolution of graphic analysis of labor. American Journal of Obstetrics and Gynecology, 132, 824–827.
- net, http://www.obgyn.net
- Sheiner, , Levy, A., Feinstein, U., Hallak, M., & Mazor, M. (2002). Risk factors and outcome of failure to progress during the first stage of labor: A population-based study. Acta Obstetrica et Gynecologia Scandinavica, 81,222–226.
- Sheiner, , Levy, A., Feinstein, U., Hershkovitz, R., Hallak, M., & Mazor, M. (2002). Obstetric risk factors for failure to progress in the first versus the second stage of labor. Journal of Maternal and Fetal Neonatal Medicine, 11, 409–413.