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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.
Labor occurs in three main stages:
First stage: This stage (which has two phases: initial and active) is labor proper. 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 into the vagina.
Second stage: The baby is delivered.
Third stage: The placenta is delivered.
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 examines the cervix to try to predict when labor will start. On average, labor lasts 12 to 18 hours in a woman’s first pregnancy and tends to be shorter, averaging 6 to 8 hours, in subsequent pregnancies.
Stages of Labor
All pregnant women should know what the main signs of the start of labor are:
A woman who has had rapid deliveries in previous pregnancies should notify her doctor as soon as she thinks she is going into labor. When 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. Contractions and back pain may be preceded or accompanied by other clues, such as the following:
Bloody show: A small discharge of blood mixed with mucus from the vagina is usually a clue that labor is about to start. The bloody show may appear as early as 72 hours before contractions start.
Rupture of membranes: 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 several 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 more than 6 weeks before the due date (prematurely, or before the 34th week), doctors do not typically induce labor until the fetus is more mature (see Premature Rupture of the Membranes (PROM)).
When the membranes rupture or when strong contractions occur less than 6 minutes apart and last 30 seconds or more, a woman should go to a hospital or birthing center. If rupture of membranes is suspected or the cervix is dilated more than 1 1/2 inches (4 centimeters), the woman is admitted. If the doctor or midwife is not sure whether labor has started, the woman is usually observed and the fetus is monitored for a hour or so, and if labor is not confirmed by then, she may be sent home.
The strength, duration, and frequency of contractions are noted. The woman's 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).
Position and presentation of the fetus affect how the fetus passes through the vagina. The most common and safest combination consists of the following:
Facing rearward (facing down when the woman lies on her back)
Face and body angled toward the right or left
Neck bent forward
Chin tucked in
Arms folded across the chest (see Figure: Position and Presentation of the Fetus)
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.
A vaginal examination is done to determine whether the membranes have ruptured and how dilated (noted in centimeters) and effaced the cervix is (noted as a percentage or in centimeters), 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).
An intravenous line is usually inserted into the woman’s arm during labor in a hospital. This line is used to give the woman fluids to prevent dehydration and, if needed, to give drugs.
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. Very rarely, vomit is inhaled, usually after general anesthesia. Inhaling vomit can cause inflammation of the lungs, which can be life threatening. Antacids are typically given to women who are having a cesarean delivery to reduce the risk of damage to the lungs if vomit is inhaled.
Soon after the woman is admitted to the hospital, the doctor or another health care practitioner listens to the fetus’s heartbeat periodically using a handheld Doppler ultrasound device, or electronic fetal heart monitoring is used to continuously monitor heartbeats.
During the first stage of labor, the heart rates of the woman and fetus are monitored periodically or continuously. Monitoring the fetus’s heart rate is the easiest way to determine whether the fetus is receiving enough oxygen. Abnormalities in the heart rate (too fast or too slow) and variations in the heart rate (over time and in response to contractions) may indicate that the fetus is in distress (see Fetal Distress). 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.
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 (given intravenously) or a particular type of anesthetic (local or regional) 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.
Analgesics may be used. 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, fentanyl or morphine is given intravenously to relieve pain. These drugs may 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. If they are given too close to delivery, the newborn may be overly sedated, making adjustment to life outside the uterus more difficult. 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. This area can be numbed by injecting a local anesthetic through the wall of the vagina into the area around the nerve that supplies sensation to the lower genital area (pudendal nerve). This procedure, called a pudendal block, is used only late in the second stage of labor, when the baby’s head is about to emerge from the vagina. It has been largely replaced by epidural injections. A more 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. The following procedures can be used:
Lumbar epidural injection is almost always used when pain relief is needed. An anesthetic is injected 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 a local anesthetic (such as bupivacaine) is continuously and slowly given through the catheter. An opioid (such as fentanyl or sufentanil) is often also injected. An epidural injection for labor and delivery does not prevent the woman from pushing. An epidural injection can also be used in cesarean deliveries.
Spinal injection 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 delivery when there are no complications.
Occasionally, use of either an epidural or a spinal injection 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. It 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 delivery because it is the quickest way to anesthetize the woman.
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