Labor
Labor is a series of rhythmic, progressive contractions of the uterus that gradually move the fetus through the lower part of the uterus (cervix) and birth canal (vagina) to the outside world.
 See the figure Stages of Labor.
Labor occurs in three main stages. The first stage (which has two phases: initial and active) is labor proper. In it, contractions cause the cervix to open gradually (dilate) and to thin and pull back (efface) until it merges with the rest of the uterus. These changes enable the fetus to pass through the vagina. The second and third stages constitute delivery of the baby and the placenta.
Labor usually starts within 2 weeks of (before or after) the estimated date of delivery. Exactly what causes labor to start is unknown. Toward the end of pregnancy (after 36 weeks), a doctor may perform a pelvic examination to try to predict when labor will start. On average, labor lasts 15 to 16 hours in a woman's first pregnancy and tends to be shorter, averaging 6 to 8 hours, in subsequent pregnancies. A woman who has had rapid deliveries in previous pregnancies should notify her doctor as soon as she thinks she is going into labor.
All pregnant women should know what the main signs of the start of labor are: contractions in the lower abdomen at regular intervals and back pain. However, other clues may precede or accompany these signs. A small discharge of blood mixed with mucus from the vagina (bloody show) is usually a clue that labor is about to start. The bloody show may appear as early as 72 hours before contractions start.
Occasionally, the fluid-filled membranes that contain the fetus (amniotic sac) rupture before labor starts, and the amniotic fluid flows out through the vagina. This event is commonly described as "the water breaks." When a woman's membranes rupture, she should contact her doctor or midwife immediately. About 80 to 90% of women whose membranes rupture before but near their due date go into labor spontaneously within 24 hours. If labor has not started after 24 hours and the baby is due, women are usually admitted to the hospital, where labor is artificially started (induced) to reduce the risk of infection. After the membranes rupture, bacteria from the vagina can enter the uterus more easily and cause an infection in the woman, the fetus, or both. Oxytocin (which causes the uterus to contract) or a similar drug, such as a prostaglandin, is used to induce labor. If the membranes rupture prematurely, doctors do not induce labor until the fetus is more mature (see Section 22, Chapter 261).
When the contractions in the lower abdomen first start, they may be weak, irregular and far apart. They may feel like menstrual cramps. As time passes, abdominal contractions become longer, stronger, and closer together. When strong contractions occur 5 minutes apart or less and the cervix is dilated more than 1½ inches (4 centimeters), the woman is admitted to the hospital or birthing center. The strength, duration, and frequency of contractions are noted. Her weight, blood pressure, heart and breathing rates, and temperature are measured, and samples of urine and blood are taken for analysis. Her abdomen is examined to estimate how big the fetus is, whether the fetus is facing rearward or forward (position), and whether the head, face, buttocks, or shoulder is leading the way out (presentation).
The presentation and position of the fetus affect how the fetus passes through the vagina. The most common and safest combination is facing rearward (toward the woman's back), with the face and body angled toward the right or left, and head first, with the neck bent forward, chin tucked in, and arms folded across the chest (see Section 22, Chapter 261). Head first is called a vertex or cephalic presentation. During the last week or two before delivery, most fetuses turn so that the back of the head presents first. If the presentation is buttocks first (breech) or shoulder first or the fetus is facing forward, delivery is considerably more difficult for the woman, fetus, and doctor. Cesarean delivery is recommended.
Usually, the vagina is examined to determine if the membranes have ruptured and how dilated and effaced the cervix is, but this examination may be omitted if the woman is bleeding or if the membranes have ruptured spontaneously. The color of the amniotic fluid is noted. The fluid should be clear and have no significant odor. If the membranes rupture and the amniotic fluid is green, the discoloration results from the fetus's first stool (fetal meconium).
Soon after the woman is admitted to the hospital, the doctor or another health care practitioner listens to the fetus's heartbeat directly using a fetal stethoscope (fetoscope) or uses an ultrasound device to monitor heartbeats (a procedure called electronic fetal heart monitoring).
During the first stage of labor, the heart rates of woman and fetus are monitored periodically or continuously. Monitoring the fetus's heart rate, with a fetal stethoscope or electronic fetal heart monitoring, is the easiest way to determine whether the fetus is receiving enough oxygen. An abnormal heart rate (too fast or too slow) may indicate that the fetus is in distress (see Section 22, Chapter 261). During the second stage of labor, the woman's heart rate and blood pressure are monitored regularly. The fetus's heart rate is monitored after every contraction or, if electronic monitoring is used, continuously.
During labor in a hospital, an intravenous line is usually inserted into the woman's arm. This line is used to give the woman fluids to prevent dehydration and, if needed, to give drugs immediately. When fluids are given intravenously, the woman does not have to eat or drink during labor, although she may choose to drink some fluids and eat some light food early in labor. An empty stomach during delivery makes the woman less likely to vomit and to inhale vomit. Inhaling vomit, although very rare, can cause respiratory distress, a potentially life-threatening disorder in which the lungs are inflamed. Usually, a woman is given an antacid by mouth to neutralize stomach acid when she is admitted to the hospital and every 3 hours after that. Antacids reduce the risk of damage to the lungs if vomit is inhaled.
 See the sidebar Monitoring the Fetus.
Pain Relief: With the advice of her doctor or midwife, a woman usually plans an approach to pain relief long before labor starts. She may choose natural childbirth, which relies on relaxation and breathing techniques to deal with pain, or she may plan to use analgesics or a particular type of anesthetic (local, regional, or general) if needed. After labor starts, these plans may be modified, depending on how labor progresses, how the woman feels, and what the doctor or midwife recommends.
A woman's need for pain relief during labor varies considerably, depending to some extent on her level of anxiety. Attending childbirth preparation classes helps prepare the woman for labor and delivery. Such preparation and emotional support from the people attending the labor tend to lessen anxiety and often markedly reduce her need for drugs to relieve pain.
If a woman requests analgesics during labor, they are usually given to her. However, because some of these drugs can slow (depress) breathing and other functions of the newborn, the amount given is as small as possible. Most commonly, meperidine or morphine is given intravenously to relieve pain. These drugs can slow the initial phase of the first stage of labor, so they are usually given during the active phase of the first stage. In addition, because these drugs have the greatest effect during the first 30 minutes after they are given, the drugs are often not given when delivery is imminent. To counteract the sedating effects of these drugs on the newborn, a doctor can give the newborn the drug naloxone immediately after delivery.
Local anesthesia numbs the vagina and the tissues around its opening. Commonly, this area is numbed by injecting a local anesthetic through the wall of the vagina and around the pudendal nerve (which supplies sensation to the lower genital area). This procedure, called a pudendal block, is used only late in labor, when the baby's head is about to emerge from the vagina. Another common but less effective procedure involves injecting a local anesthetic at the opening of the vagina. With both procedures, the woman can remain awake and push, and the fetus's functions are unaffected. These procedures are useful for deliveries that have no complications.
Regional anesthesia numbs a larger area. It may be used for women who want more complete pain relief. A lumbar epidural injection is almost always used. This procedure involves injecting an anesthetic in the lower back--into the space between the spine and the outer layer of tissue covering the spinal cord (epidural space). Alternatively, a catheter is placed in the epidural space, and opioids, such as fentanyl and sufentanil, are continuously and slowly given through the catheter. Another procedure (spinal anesthesia) involves injecting an anesthetic into the space between the middle and inner layers of tissue covering the spinal cord (subarachnoid space). A spinal injection is typically used for cesarean sections when there are no complications. Neither an epidural nor a spinal injection prevents the woman from pushing adequately. Occasionally, use of either procedure causes a fall in blood pressure. Consequently, if one of these procedures is used, the woman's blood pressure is measured frequently.
General anesthesia makes a woman temporarily unconscious. This method is rarely necessary and infrequently used because it may slow the function of the fetus's heart, lungs, and brain. Although this effect is usually temporary, it can interfere with the newborn's adjustment to life outside the uterus. General anesthesia is typically used for emergency cesarean sections because it is the quickest way to anesthetize the woman.
See the sidebar Natural Childbirth.
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