Preterm Labor and Birth

In general, a normal human pregnancy is about 40 weeks long (9.2 months). Health care providers now define “full-term” birth as birth that occurs between 39 weeks and 40 weeks and 6 days of pregnancy.¹ Infants born during this time are considered full-term infants.

Infants born in the 37th and 38th weeks of pregnancy—previously called term but now referred to as “early term”—face more health risks than do those born at 39 or 40 weeks.²

Deliveries before 37 weeks of pregnancy are considered “preterm” or premature:

• Labor that begins before 37 weeks of pregnancy is preterm or premature labor.
• A birth that occurs before 37 weeks of pregnancy is a preterm or premature birth.
• An infant born before 37 weeks in the womb is a preterm or premature infant. (These infants are commonly called “preemies” as a reference to being born prematurely.)

“Late preterm” refers to 34 weeks through 36 weeks of pregnancy. Infants born during this time are considered late-preterm infants, but they face many of the same health challenges as preterm infants. More than 70% of preterm infants are born during the late-preterm time frame.³

Preterm birth is the most common cause of infant death and is the leading cause of long-term disability in children.⁴ Many organs, including the brain, lungs, and liver, are still developing in the final weeks of pregnancy. The earlier the delivery, the higher the risk of serious disability or death.

Infants born prematurely are at risk for cerebral palsy (a group of nervous system disorders that affect control of movement and posture and limit activity), developmental delays, and vision and hearing problems.

Late-preterm infants typically have better health outcomes than those born earlier, but they are still three times more likely to die in the first year of life than are full-term infants.³ Preterm births can also take a heavy emotional and economic toll on families.⁵

Preterm labor is any labor that occurs from 20 weeks through 36 weeks of pregnancy. Here are the symptoms¹:

• Contractions (tightening of stomach muscles, or birth pains) every 10 minutes or more often
• Change in vaginal discharge (leaking fluid or bleeding from the vagina)
• Feeling of pressure in the pelvis (hip) area
• Low, dull backache
• Cramps that feel like menstrual cramps
• Abdominal cramps with or without diarrhea

It is normal for pregnant women to have some uterine contractions throughout the day. It is not normal to have frequent uterine contractions, such as six or more in one hour. Frequent uterine contractions, or tightenings, may cause the cervix to begin to open.

If a woman thinks that she might be having preterm labor, she should call her doctor or go to the hospital to be evaluated.

The causes of preterm labor and premature birth are numerous, complex, and only partly understood. Medical, psychosocial, and biological factors may all play a role in preterm labor and birth.

There are three main situations in which preterm labor and premature birth may occur:

• Spontaneous preterm labor and birth. This term refers to unintentional, unplanned delivery before the 37th week of pregnancy. This type of preterm birth can result from a number of causes, such as infection or inflammation, although the cause of spontaneous preterm labor and delivery is usually not known. A history of delivering preterm is one of the strongest predictors for subsequent preterm births.¹
• Medically indicated preterm birth. If a serious medical condition—such as preeclampsia—exists, the health care provider might recommend a preterm delivery. In these cases, health care providers often take steps to keep the baby in the womb as long as possible to allow for additional growth and development, while also monitoring the mother and fetus for health issues. Providers also use additional interventions, such as steroids, to help improve outcomes for the baby.
• Non-medically indicated (elective) preterm delivery. Some late-preterm births result from inducing labor or having a cesarean delivery even though there is not a medical reason to do so, even though this practice is not recommended. Research indicates that even babies born at 37 or 38 weeks of pregnancy are at higher risk for poor health outcomes than are babies born at 39 weeks of pregnancy or later. Therefore, unless there are medical problems, health care providers should wait until at least 39 weeks of pregnancy to induce labor or perform a cesarean delivery to prevent possible health problems.²
  
  The National Child and Maternal Health Education Program, led by NICHD in collaboration with 33 other agencies, organizations, and groups focused on maternal and child health, offers videos and other information about why it’s best to wait until at least 39 weeks of pregnancy to deliver unless there is a medical reason. Learn more about the “Is It Worth It?” initiative.

There are several risk factors for preterm labor and premature birth, including ones that researchers have not yet identified. Some of these risk factors are "modifiable," meaning they can be changed to help reduce the risk. Other factors cannot be changed.

Health care providers consider the following factors to put women at high risk for preterm labor or birth:

• Women who have delivered preterm before, or who have experienced preterm labor before, are considered to be at high risk for preterm labor and birth.¹
• Being pregnant with twins, triplets, or more (called "multiple gestations") or the use of assisted reproductive technology is associated with a higher risk of preterm labor and birth. One study showed that more than 50% of twin births occurred preterm, compared with only 10% of births of single infants.²
• Women with certain anomalies of the reproductive organs are at greater risk for preterm labor and birth than are women who do not have these anomalies. For instance, women who have a short cervix (the lower part of the uterus) or whose cervix shortens in the second trimester (fourth through sixth months) of pregnancy instead of the third trimester are at high risk for preterm delivery.

Certain medical conditions, including some that occur only during pregnancy, also place a woman at higher risk for preterm labor and delivery. Some of these conditions include³:

• Urinary tract infections
• Sexually transmitted infections
• Certain vaginal infections, such as bacterial vaginosis and trichomoniasis
• High blood pressure
• Bleeding from the vagina
• Certain developmental anomalies in the fetus
• Pregnancy resulting from in vitro fertilization
• Being underweight or obese before pregnancy
• Short time period between pregnancies (less than 6 months between a birth and the beginning of the next pregnancy)
• Placenta previa, a condition in which the placenta grows in the lowest part of the uterus and covers all or part of the opening to the cervix
• Being at risk for rupture of the uterus (when the wall of the uterus rips open). Rupture of the uterus is more likely if you have had a prior cesarean delivery or have had a uterine fibroid removed.
• Diabetes (high blood sugar) and gestational diabetes (which occurs only during pregnancy)
• Blood clotting problems

Other factors that may increase risk for preterm labor and premature birth include:

• Ethnicity. Preterm labor and birth occur more often among certain racial and ethnic groups. For example, infants of African American mothers are more likely to be born preterm than infants of white mothers. American Indian/Alaska Native mothers are also more likely to give birth preterm than are white mothers.⁴
• Age of the mother.
  • Women younger than age 18 are more likely to have a preterm delivery.
  • Women older than age 35 are also at risk of having preterm infants because they are more likely to have other conditions (such as high blood pressure and diabetes) that can cause complications requiring preterm delivery.⁴
• Certain lifestyle and environmental factors, including:³
• Late or no health care during pregnancy
• Smoking
• Drinking alcohol
• Using illegal drugs
• Domestic violence, including physical, sexual, or emotional abuse
• Lack of social support
• Stress
• Long working hours with long periods of standing
• Exposure to certain environmental pollutants

Currently, there is no definitive way to predict preterm labor or premature birth. Many research studies are focusing on this important issue. By identifying which women are at increased risk, health care providers may be able to provide early interventions, treatments, and close monitoring of these pregnancies to prevent preterm delivery or to improve health outcomes.

However, in some situations, health care providers know that a preterm delivery is very likely. Some of these situations are described in the following sections.

Shortened Cervix

As a preparation for birth, the cervix (the lower part of the uterus) naturally shortens late in pregnancy. However, in some women, the cervix shortens prematurely, around the fourth or fifth month of pregnancy, increasing the risk for preterm delivery.

In some cases, a health care provider may recommend measuring a pregnant woman’s cervical length, especially if she previously had preterm labor or a preterm birth. Ultrasound scans may be used to measure cervical length and identify women with a shortened cervix.¹

"Incompetent" Cervix

The cervix normally remains closed during pregnancy. In some cases, the cervix starts to open early, before a fetus is ready to be born. Health care providers may refer to a cervix that begins to open as an "incompetent" cervix. The process of cervical opening is painless and unnoticeable, without labor contractions or cramping.²

To try to prevent preterm birth, a doctor may place a stitch around the cervix to keep it closed. This procedure is called cervical cerclage. NICHD-supported research has found that, in women with a prior preterm birth who have a short cervix, cerclage may improve the likelihood of a full-term delivery.³

If a woman is concerned that she could be showing signs of preterm labor, she should call her health care provider or go to the hospital to be evaluated. In particular, a woman should call if she has more than six contractions in an hour or if fluid or blood is leaking from the vagina.

Physical Exam

If a woman is experiencing signs of labor, the health care provider may perform a pelvic exam to see if:

• The membranes have ruptured
• The cervix is beginning to get thinner (efface)
• The cervix is beginning to open (dilate)

Any of these situations could mean the woman is in preterm labor.

Providers may also do an ultrasound exam and use a monitor to electronically record contractions and the fetal heart rate.

Fetal Fibronectin (fFN) Test

This test is used to detect whether the protein fetal fibronectin is being produced. fFN is like a biological “glue” between the uterine lining and the membrane that surrounds the fetus.¹

Normally fFN is detectable in the pregnant woman's secretions from the vagina and cervix early in the pregnancy (up to 22 weeks, or about 5 months) and again toward the end of the pregnancy (1 to 3 weeks before labor begins). It is usually not present between 24 and 34 weeks of pregnancy (5½ to 8½ months). If fFN is detected during this time, it may be a sign that the woman may be at risk of preterm labor and birth.

In most cases, the fFN test is performed on women who are showing signs of preterm labor. Testing for fFN can help predict which pregnant women showing signs of preterm labor will have a preterm delivery.² It is typically used for its negative predictive value, meaning that if it is negative, it is unlikely that a woman will deliver within the next 7 days.

Currently, treatment options for preventing preterm labor or birth are somewhat limited, in part because the cause of preterm labor or birth is often unknown. But there are a few options, described in the following sections.

Hormone treatment. Progesterone, a hormone produced by the body during pregnancy, was thought to prevent preterm birth in certain groups at high risk of preterm birth, such as those with a prior preterm birth. NICHD’s Maternal-Fetal Medicine Units Network found that progesterone given to women at risk of preterm birth due to a prior preterm birth reduces chances of a subsequent preterm birth by one-third, when started at 16 weeks of gestation and continued to 37 weeks of gestation.^1,2 Because subsequent research did not show the same effect, use of progesterone to prevent preterm birth is now under review by the U.S. Food and Drug Administration.

Cerclage. A surgical procedure called cervical cerclage is sometimes used to try to prevent early labor in women who have an incompetent (weak) cervix and have experienced early pregnancy loss accompanied by a painless opening (dilation) of the cervix (the bottom part of the uterus). In the cerclage procedure, a doctor stitches the cervix closed. The stitch is then removed closer to the woman's due date.

Bed rest. Contrary to expectations, confining the mother to bed rest does not help to prevent preterm birth. In fact, bed rest can make preterm birth even more likely among some women.^3,4

Women should discuss all of their treatment options—including the risks and benefits—with their health care providers. If possible, these discussions should occur during regular prenatal care  visits, before there is any urgency, to allow for a complete discussion of all the issues.

If a pregnant woman is showing signs of preterm labor, her doctor will often try treatments to stop labor and prolong the pregnancy until the fetus is more fully developed. Treatments include therapies to try to stop labor (tocolytics) and medications administered before birth to improve outcomes for the infant if born preterm (antenatal steroids to improve the respiratory outcomes and neuroprotective medications such as magnesium sulfate).

Medications to Delay Labor

Drugs called tocolytics (pronounced toh-coh-LIT-iks) can be given to many women with symptoms of preterm labor. These drugs can slow or stop contractions of the uterus and may prevent labor for 2 to 7 days. One common treatment for delaying labor is magnesium sulfate (pronounced mag-NEEZ-ee-um SUL-fate), given to the pregnant woman intravenously through a needle inserted in an arm vein.

Medications to Speed Development of the Fetus

Tocolytics may provide the extra time for treatment with corticosteroids (pronounced kohr-tuh-koh-STER-oids) to speed up development of the fetus's lungs and some other organs or for the pregnant woman to get to a hospital that offers specialized care for preterm infants. Corticosteroids can be particularly effective if the pregnancy is between 24 and 34 weeks (between 5½ and 7¾ months) and the woman's health care provider suspects that the birth may occur within the next week.¹ Intravenously delivered magnesium sulfate may also reduce the risk of cerebral palsy if the child is born early.²

Researchers have found that some methods for trying to stop preterm labor are not as effective as once thought. These include:

• Home uterine monitors¹
• Routine screening of all asymptomatic women for bacterial vaginosis (Trichomonas vaginalis) infection.² Routine screening and treatment with antibiotics did not reduce preterm birth; in fact, the latter increased the risk of preterm birth.²