Merck Manual

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Raul Artal-Mittelmark

, MD, Saint Louis University School of Medicine

Reviewed/Revised May 2021 | Modified Sep 2022
Topic Resources

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. Standing and walking during the first stage of labor can shorten it by more than 1 hour.

Stages of Labor


From the beginning of labor to the full opening (dilation) of the cervix—to about 4 inches (10 centimeters).

Initial (Latent) Phase

  • Contractions are irregular at first but become progressively stronger and more rhythmic.

  • Discomfort is minimal.

  • The cervix begins to thin and opens to about 1 1/2 inches (4 centimeters).

  • The initial phase lasts an average of 8 hours (usually no more than 20 hours) in a first pregnancy and 5 hours (usually no more than 12 hours) in subsequent pregnancies.

Stages of Labor

Active Phase

  • The cervix opens from about 1 1/2inches (4 centimeters) to the full 4 inches (10 centimeters). It thins and pulls back (effaces) until it merges with the rest of the uterus.

  • The presenting part of the baby, usually the head, begins to descend into the woman’s pelvis.

  • The woman begins to feel the urge to push as the baby descends, but she should resist it. Pushing too early may be unnecessarily tiring and frequently tears the cervix, which then must be repaired.

  • This phase averages about 5 to 7 hours in a first pregnancy and 2 to 4 hours in subsequent pregnancies.

Stages of Labor


From the complete opening of the cervix to delivery of the baby: This stage usually lasts about 2 hours in a first pregnancy and about 1 hour in subsequent pregnancies. It may last another hour or more if the woman has been given an epidural injection or a drug to relieve pain. During this stage, the woman pushes.

Stages of Labor


From delivery of the baby to delivery of the placenta: This stage usually lasts only a few minutes but may last up to 30 minutes.

Stages of Labor

Start of labor

All pregnant women should know what the main signs of the start of labor are:

  • Contractions in the lower abdomen at regular intervals

  • Back pain

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:

If a woman’s membranes rupture before labor starts, she should contact her doctor or midwife immediately or be taken to the nearest birthing center. About 80 to 90% of women whose membranes rupture at or 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 Induction of Labor Induction of labor is the artificial starting of labor. Usually, labor is induced by giving the woman oxytocin, a drug that makes the uterus contract more frequently and more forcefully. The... read more ) 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.

After a woman with premature rupture of membranes is admitted to a birthing center, oxytocin (which causes the uterus to contract) or a similar drug, such as a prostaglandin, is used to induce labor. However, 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.

Admission to a hospital or birthing center

A woman should go to a hospital or birthing center when one of the following occurs:

  • The membranes rupture.

  • Strong contractions occur 6 minutes apart or less and last 30 seconds or more.

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 Vaginal Exam in Labor

When the woman is admitted, 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:

  • Head first

  • 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

Normal Position and Presentation of the Fetus

Toward the end of pregnancy, the fetus moves into position for delivery. Normally, the position of a fetus is facing rearward (toward the woman’s back) with the face and body angled to one side and the neck flexed, and presentation is head first.

Normal Position and Presentation of the Fetus

A vaginal examination using a speculum is done to determine whether the membranes have ruptured. (A speculum is a metal or plastic instrument that spreads the walls of the vagina apart). Then the doctor or midwife examines the vagina and cervix with a hand to determine how dilated (noted in centimeters) and how pulled back (effaced) the cervix is (noted as a percentage or in centimeters). 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.

Fetal monitoring

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 type of stethoscope (fetoscope) or a handheld Doppler ultrasound device or continuously using electronic fetal heart monitoring. Practitioners monitor the fetus's heart to determine whether the fetus's heart rate is normal and thus whether the fetus is in distress Fetal Distress Fetal distress refers to signs before and during childbirth indicating that the fetus is not well. Fetal distress is an uncommon complication of labor. It typically occurs when the fetus has... read more . Certain abnormal changes in the fetus’s heart rate during contractions can indicate that the fetus is not receiving enough oxygen.

The fetus’s heart rate can be monitored in the following ways:

  • Externally: An ultrasound device (which transmits and receives ultrasound waves) is attached to the woman’s abdomen. Or a fetoscope is placed on the woman's abdomen at regular intervals.

  • Internally: An electrode (a small round sensor attached to a wire) is inserted through the woman’s vagina and attached to the fetus’s scalp. The internal approach is typically used when problems during labor appear likely or when signals detected by the external device cannot be recorded. This approach can be used only after the membranes that contain the fetus have ruptured (described as "the water breaks").

Use of an external ultrasound device or internal electrode to monitor the fetus's heart rate is called electronic fetal monitoring. Electronic monitoring is used to continuously monitor the contractions of the uterus. It is used for virtually all high-risk pregnancies and, in many practices, for all pregnancies.

In a high-risk pregnancy Overview of High-Risk Pregnancy There is no formal or universally accepted definition of a high-risk pregnancy. Generally, however, a high-risk pregnancy involves at least one of the following: The woman or baby is more likely... read more , electronic monitoring is sometimes used as part of a nonstress test, in which the fetus’s heart rate is monitored as the fetus lies still and as it moves. If the heart rate does not speed up as expected on two occasions within 20 minutes of when the fetus moves, the heart rate is described as nonreactive or nonreassuring. Then an ultrasound biophysical profile may be done to check on the fetus's well-being.

For an ultrasound biophysical profile, ultrasonography is used to produce images of the fetus in real time, and the fetus is observed. After 30 minutes, doctors assign a score of 0 or 2 to the following:

  • Results of the nonstress test (reactive or nonreactive)

  • Amount of amniotic fluid

  • Presence or absence of a period of rhythmic breathing

  • Presence or absence of at least three clearly visible movements of the fetus

  • Muscle tone of the fetus, indicated by stretching, then flexing the fingers, a limb, or the trunk

A score of up to 10 is possible.

Based on the result, doctors may allow labor to continue or may do a cesarean delivery immediately.

During the first stage of labor, the heart rate of the fetus is monitored periodically with a stethoscope or an ultrasound device or continuously using electronic monitoring. 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 (fetal distress Fetal Distress Fetal distress refers to signs before and during childbirth indicating that the fetus is not well. Fetal distress is an uncommon complication of labor. It typically occurs when the fetus has... read more ). The heart rate of the woman is also monitored periodically.

During the second stage of labor, the fetus’s heart rate is monitored after every contraction or, if electronic monitoring is used, continuously. The woman’s heart rate and blood pressure are monitored regularly.

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 one of the following:

  • Natural childbirth, which relies on relaxation and breathing techniques to deal with pain

  • Analgesics (given intravenously)

  • 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.

Analgesics (pain relievers) 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, an opioid Opioid Pain Relievers Pain relievers (analgesics) are the main drugs used to treat pain. Doctors choose a pain reliever based on the type and duration of pain and on the drug's likely benefits and risks. Most pain... read more such as 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 opioid antidote naloxone Emergency treatment 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 and does not make women more likely to need a cesarean delivery. 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 in the woman. 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.

Natural childbirth

Natural childbirth uses relaxation and breathing techniques to control pain during childbirth.

To prepare for natural childbirth, a pregnant woman and her partner take childbirth classes, usually six to eight sessions over several weeks, to learn how to use the relaxation and breathing techniques. They also learn what happens in the various stages of labor and delivery.

The relaxation technique involves consciously tensing a part of the body and then relaxing it. This technique helps a woman relax the rest of her body while the uterus is contracting during labor and relax her whole body between contractions.

The breathing technique involves several types of breathing, which are used at different times during labor. During the first stage of labor, before the woman begins to push, the following types of breathing may help:

  • Deep breathing with slow exhalation to help the woman relax at the beginning and end of a contraction

  • Fast, shallow breathing (panting) in the upper chest at the peak of a contraction

  • A pattern of panting and blowing to help the woman refrain from pushing when she has an urge to push before the cervix is completely open (dilated) and pulled back (effaced)

The woman and her partner should practice relaxation and breathing techniques regularly during pregnancy. During labor, the woman’s partner can help her by reminding her of what she should be doing at a particular stage and by noticing when she is tense, in addition to providing emotional support. The partner may massage the woman to help her relax more.

The most well-known method of natural childbirth is probably the Lamaze method. Another method, the Leboyer method, includes birth in a darkened room and immersion of the baby into lukewarm water immediately after delivery.

Drugs Mentioned In This Article

Generic Name Select Brand Names
ABSTRAL, Actiq, Duragesic, Fentora, IONSYS, Lazanda, Onsolis, Sublimaze, SUBSYS
ARYMO ER, Astramorph PF, Avinza, DepoDur, Duramorph PF, Infumorph, Kadian, MITIGO, MORPHABOND, MS Contin, MSIR, Opium Tincture, Oramorph SR, RMS, Roxanol, Roxanol-T
EVZIO, Kloxxado, Narcan, ZIMHI
Marcaine, Marcaine Spinal, POSIMIR, Sensorcaine, Sensorcaine MPF , Xaracoll
DSUVIA, Sufenta
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