Preterm 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.1Infants 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


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


Exams and Tests

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 (pronounced fy-broh-NEK-tun) 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 predict with about 50% accuracy which pregnant women showing signs of preterm labor are likely to 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 below.

A progesterone medication (17-alpha hydroxyprogesterone caproate, or 17P) may prevent preterm birth among women who have had a prior preterm birth. The U.S. Food and Drug Administration (FDA) approved hydroxyprogesterone caproate injection (Makena™) to reduce the risk of preterm delivery in pregnant women with a history of delivering early. Consult with your health care provider to learn more about this and other drugs, including whether they are safe for use during pregnancy.

Hormone treatment. The only preventive drug therapy is progesterone (pronounced proh-JES-tuh-rohn), a hormone produced by the body during pregnancy, which is given to women at risk of preterm birth, such as those with a prior preterm birth. The 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. This preventive therapy is given beginning at 16 weeks of gestation and continues to 37 weeks of gestation.1,2 The treatment works among all ethnic groups and can improve outcomes for infants.

Cerclage. A surgical procedure called cervical cerclage (pronouncedsair-KLAZH) is sometimes used to try to prevent early labor in women who have an incompetent (weak) cervix and have experienced earlypregnancy 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.

Possible Complications

A developing baby goes through important growth during the final weeks and months of pregnancy. Many organ systems, including the brain, lungs, and liver need the final weeks of pregnancy to fully develop. Read Your Baby Grows Throughout Your Entire Pregnancy[PDF-312KB]. There is a higher risk of serious disability or death the earlier the baby is born. Some problems that a baby born too early may face include—


Preventing preterm birth remains a challenge because the causes of preterm births are numerous, complex, and not always well understood. However, pregnant women can take important steps to help reduce their risk of preterm birth and improve their general health.

  • Quit smoking. For help in quitting, call 1-800-QUIT-NOW (1-800-784-8669) or visit Tobacco Use and Pregnancy: Resources
  • Avoid alcohol and illicit drugs
  • Get prenatal care as soon as you think you may be pregnant and throughout the pregnancy
  • Seek medical attention for any warning signs or symptoms of preterm labor