A normal pregnancy lasts 37 to 42 weeks, counting from the first day of the last menstrual period. A pregnancy that continues beyond 37 weeks is called a "term" pregnancy. Preterm labor is defined as labor that begins before 37 weeks of pregnancy.
Approximately 12 percent of babies in the United States are born preterm; 80 percent of these are due to preterm labor that occurs on its own or after preterm premature rupture of the fetal membranes (or "broken bag of waters"). The remaining 20 percent are planned early deliveries that are done for maternal or fetal problems that prevent the woman from being able to safely continue with her pregnancy.
Not all women who have preterm labor will deliver their baby early; estimates are that between 30 and 50 percent of women who develop preterm labor will go on to deliver their infant at term. If preterm labor leads to an early delivery, the premature newborn is at risk for problems related to incomplete development of its organ systems. These problems include difficulty with breathing, staying warm, feeding, as well as injury to the eyes, intestines, and nervous system.
Preterm birth is a major cause of newborn complications and death. Regular prenatal care can help to identify some, but not all, women at risk for preterm labor. Should preterm labor occur, measures can be taken to delay delivery and decrease the risk of newborn complications.
PRETERM LABOR RISK FACTORS
It is difficult to predict who will develop preterm labor. Certain obstetrical conditions and other factors are known to increase a woman's risk. However, most preterm births occur in women who have no known risk factors.
The strongest risk factor for preterm birth is a previous preterm birth, although most women who have had a preterm birth will have a term pregnancy in the future. As an example, one study found that only 22 percent of women with a previous preterm delivery had a preterm delivery with their next pregnancy .
Other factors that may increase a woman's risk include:
- Being pregnant with twins, triplets, or more
- A history of cervical surgery (eg, conization or cone biopsy) for abnormal Pap smears, if the amount of the cervix removed is large
- Abnormalities of the uterus
- Uterine bleeding, especially in the second or third trimester
- Use of certain illicit drugs, such as cocaine
- Cigarette smoking
- Some infections
- Low prepregnancy weight and low weight gain during pregnancy
- Excessive amniotic fluid
- Moderate to severe anemia early in the pregnancy
- A short interval (less than 12 to 18 months) between pregnancies (deliveries)
- Abdominal surgery during pregnancy
Black women appear to have double the incidence of preterm labor and delivery when compared to white women. The risk of preterm delivery is also higher in women under 18 to 20 years of age. Older maternal age alone (over 35 to 40) is not associated with an increased risk of preterm labor. However, older women are more likely to have other conditions (such as hypertension and diabetes) that can cause complications requiring preterm delivery.
PRETERM LABOR CAUSES
It is usually difficult to identify the cause of preterm labor. Four general categories causes include:
Uterine bleeding — Conditions like placenta previa (when the placenta partially or completely covers the cervix) and placental abruption (when the placenta separates from the uterus before delivery) can cause the fetal membranes to rupture prematurely and can trigger preterm labor.
Stretching of the uterus — Having twins, triplets, or more, or having polyhydramnios (an excessive amount of amniotic fluid around the baby) causes stretching of the uterus, which can lead to uterine contractions and preterm labor.
Bacteria or inflammation — Bacteria or inflammation caused by an infection in the uterus can stimulate the production of substances that trigger uterine contractions.
Physical or psychological stress — Severe stress can lead to the release of hormones that cause uterine contractions and preterm labor.
PREDICTING PRETERM DELIVERY
Research is ongoing to identify a chemical or physical marker that predicts whether and when premature delivery will occur. Two tests have been identified that may be helpful in some settings.
Fetal fibronectin — A substance called fetal fibronectin is released when the fetal membranes begin to change prior to labor. Studies have shown that if this substance is not present in vaginal discharge in high concentrations, premature delivery is unlikely. If there are high amounts of fetal fibronectin, it does not always mean that the woman will deliver prematurely.
Cervical length — Ultrasound measurement of the cervix can help to predict the risk of preterm delivery; the risk increases as cervical length decreases.
PRETERM LABOR SIGNS AND SYMPTOMS
The signs of preterm labor are similar to the signs of labor at the end of pregnancy:
- Change in type or amount of vaginal discharge (watery, mucus, or bloody)
- Pelvic or lower abdominal pressure or pain
- Constant, low, dull backache
- Mild or menstrual-like abdominal cramps, with or without diarrhea
- Regular or frequent contractions or uterine tightening that may be painless
- Ruptured membranes (broken water)
Braxton Hicks contractions (also called false labor contractions) are uterine contractions (tightening of the uterus) that occur less than eight times in an hour or four times every twenty minutes; these contractions are not accompanied by bleeding or vaginal discharge and are relieved by resting. These are normal and do not increase the risk of preterm birth. However, it is often difficult to tell the difference between preterm labor and false labor without having a pelvic examination.
In the early stages of labor, a woman may experience cramping that is relatively mild and occurs irregularly. At this stage the discomfort may be similar to menstrual cramping and may cause low back pain. As uterine contractions strengthen they usually become more painful and occur at regular and shorter intervals.
In addition, a woman may notice excessive mucus discharge from the vagina. Light bleeding or spotting is also common. The fetal membranes can rupture (known as "water breaking") before or during preterm labor. If this happens, a trickle to a sudden gush of fluid will drain from the vagina.
A woman should contact her hospital or healthcare provider immediately if she is concerned she could be in preterm labor or has other concerning symptoms. In particular, a woman should call if she has more than six contractions in an hour that continue despite lying down, if she has leakage of amniotic fluid, or has any vaginal bleeding.
The healthcare provider will perform a pelvic examination to determine if the membranes have ruptured and if the cervix is effacing (thinning) or dilating (beginning to open), and may also perform an ultrasound examination. He or she may also place a monitor on the uterus that electronically records uterine contractions and the fetal heart rate.
A healthcare provider may ask a woman to monitor herself for contractions. This is best accomplished by lying down and gently feeling the uterus with the fingertips. Normally, the uterus should be relaxed, soft, and easily indented by pushing on it with the fingers. During a contraction, the uterus becomes firm and difficult to indent. The time between the start of one contraction and the start of the next indicates how often contractions are occurring.
PRETERM LABOR TREATMENT
Treatment can be given in an attempt to slow or stop preterm labor. The primary goal of treatment is to delay delivery long enough that steroids, which promote development of the baby's lungs, can be given. Delaying preterm delivery also allows the woman to be transferred, if necessary, to a facility that can provide specialized care to a premature infant.
Treatment to delay delivery is typically recommended if the woman is less than 34 weeks pregnant because infants born before 34 weeks are at particularly high risk for complications of premature birth. However, if the mother or infant's health are at risk, labor may be allowed to proceed. Labor may also be allowed to proceed if the mother is more than 34 weeks pregnant or if tests show that the baby's lungs are fully developed.
A woman in preterm labor will be admitted to the hospital for close monitoring while medications to stop labor are administered. An intravenous line will be inserted to give medications and fluids, and a fetal monitor will be used to measure uterine contractions and the baby's heart rate.
Treatments to stop labor — If the mother and baby are healthy, medications are often used to try to relax the uterine muscle and stop contractions. Medications used to stop or slow labor are called "tocolytic" agents. They include terbutaline, magnesium sulfate, nifedipine, and indomethacin. Some of these drugs are given intravenously or by injection while others can be taken orally.
Tocolytics are usually given along with a steroid (glucocorticoid) injection (see below). Tocolytic medications are intended to delay delivery for several hours and optimally for 48 hours.
While tocolytics are used, the mother is monitored for medication side effects. If labor stops, the woman is usually kept in the hospital for a period of time to monitor for more uterine contractions. Depending upon a number of factors, the woman may be discharged home or asked to stay in the hospital. While at home, the woman may be asked to limit her activities, and she should contact her hospital or healthcare provider immediately if signs of labor return.
Treatments to help the infant — Steroids (glucocorticoids) can speed the development of a preterm infant's lungs, and are often administered during preterm labor. Steroids help the lungs mature and may promote the production of surfactant, a substance that prevents the collapse of alveoli (small sacs in the lungs where air is exchanged). Steroids also decrease the infant's risk for intraventricular hemorrhage (bleeding into the brain) and other complications affecting the bowels and circulatory system.
Steroids are usually administered if the mother is between about 23 and 34 weeks of gestation. Before about 23 weeks of gestation, the fetus is too immature to benefit from steroids. After 34 weeks, the infant's lungs are usually sufficiently developed, although steroids may be given if fetal lung tests suggest that the lungs are not mature. The most commonly used steroid is betamethasone.
The timing of the dose of steroids is important. Steroids must be given to the mother as an injection several hours before the infant is delivered. A second dose is usually given 24 hours after the first dose. There is probably some benefit from steroids, even if the woman delivers before the second dose is given. The greatest benefit is seen when the steroid is given at least 48 hours before the infant is delivered. It is not usually necessary to repeat the steroid treatment later in pregnancy if preterm labor recurs.
If the mother delivers early, a number of treatments can be given to support the premature infant. Over the past decade, significant advances have been made in the care of premature newborns. However, not all hospitals are equipped to care for them. For this reason, it is important that a woman who is at high risk for premature delivery be treated in a hospital with a neonatal intensive care unit.
PRETERM LABOR PREVENTION
One of the most important things a pregnant woman can do to prevent preterm labor is to stop habits that can be harmful, such as smoking and use of illegal drugs.
Women with a history of a previous preterm birth at less than 37 weeks of pregnancy (due to spontaneous labor or premature rupture of membranes) may be offered a progesterone supplement, either as an injection or a vaginal gel, to prevent recurrent preterm labor. Progesterone supplementation is begun between 16 and 26 weeks of pregnancy and continued until 36 weeks. There is no evidence that this drug is effective in women with no previous history of preterm labor. In addition, it has not been effective in women with multiple gestations (eg, twins).
WHERE TO GET MORE INFORMATION
Your healthcare provider is the best source of information for questions and concerns related to your medical problem.
This article will be updated as needed on our web site (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.
Patient level information — UpToDate offers two types of patient education materials.
The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.
Patient information: Preterm labor (The Basics)
Patient information: Preterm premature rupture of membranes (The Basics)
Patient information: Activity during pregnancy (The Basics)
Patient information: Having twins (The Basics)
Patient information: Hyperthyroidism (overactive thyroid) and pregnancy (The Basics)
Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.
This topic currently has no corresponding Beyond the Basics content.
Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.
Antenatal corticosteroid therapy for reduction of neonatal morbidity and mortality from preterm delivery
Inhibition of acute preterm labor
Management of pregnant women after inhibition of acute preterm labor
Overview of preterm labor and birth
Pathogenesis of spontaneous preterm birth
Prevention of spontaneous preterm birth
Risk factors for preterm labor and delivery
The following organizations also provide reliable health information.
- National Library of Medicine
- National Institute of Child Health and Human Development