Labor

Labor is the process that begins with uterine contractions to cause dilation of the cervix and ends with the delivery of both the baby and the placenta. It may be induced medicinally or be spontaneous.
==Stages of labor==
There are three stages of labor.
===First stage===
The first stage covers the period from the beginning of uterine contractions to the point where the cervix is fully dilated, or open to around ten centimeters. For a first birth, this period is around twelve hours long, but that time is cut to eight hours in subsequent births.
A sign of impending labor is the appearance of a small amount of blood-tinged mucus from the vagina.
===Second stage===
The second stage covers the period from complete dilation to birth. The average amount of time for this stage is fifty minutes for first births and twenty minutes for subsequent births. During this stage, labor pains are intense and last from about one to one and a half minutes every two or three minutes.
The bag of membranes occurs during the early part of this stage — colloquially, this is referred to as 'water breaking'. The muscles of the abdomen begin contracting involuntarily and the patient directs all her strength to bearing down through the contractions. As labor continues, the scalp of the fetus appears from the vulvar opening and recedes and advances bit by bit until the head is visible.
Once crowning occurs, the obstetrician can make the decision regarding whether or not to cut the perineum, or the area between the anus and vulva, to facilitate labor. The head is fully delivered and the shoulders come immediately after along with a gush of amniotic fluid.
===Third stage===
The final stage takes place from the birth of the baby to the expulsion of the placenta. After the fetus has been delivered, the rest of the amniotic fluid follows. Weaker contractions begin and by about ten minutes, the placenta is delivered followed by bleeding from the uterus.
==Special circumstances==
Not all labors are normal. The following are some of the abnormal presentations of labor.
*Arrested labor is the failure of labor to proceed.
*Back labor is labor where the fetal head is placed with the skull facing the mother's sacrum.
*Hypertonic labor is a condition where there are frequent and very painful contractions but they are very poor quality and unable to encourage the dilation of the cervix.
*Hypotonic labor is a condition in which very few contractions take place.
*Missed labor is when true labor pains start but then subside, usually because of death of the fetus. 
*Obstructed labor is labor in which there is interference with fetal descent.
*Precipitate labor is labor that takes place very quickly, the time between dilation and delivery being about three hours.
*Preterm labor is labor that begins before 37 weeks of gestation.
*Prolonged labor is labor lasting more than twenty hours.
See also: Premature labor