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. 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.

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.

Preterm birth is the most common cause of infant death and is the leading cause of long-term disability in children. 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. Preterm births can also take a heavy emotional and economic toll on families.

Preterm labor is labor that starts before 37 completed weeks of pregnancy. It can lead to premature birth. Premature babies may face serious health risks.

Symptoms of preterm labor include

• Contractions every 10 minutes or more often
• Leaking fluid or bleeding from the vagina
• Feeling of pressure in the pelvis
• Low, dull backache
• Cramps that feel like menstrual cramps
• Abdominal cramps with or without diarrhea

If you think you might be having preterm labor, contact your health care provider.

Preterm birth is when a baby is born too early, before 37 weeks of pregnancy have been completed. In 2022, preterm birth affected about 1 of every 10 infants born in the United States. The preterm birth rate declined 1% in 2022 to 10.4%, following an increase of 4% from 2020 to 2021. However, racial and ethnic differences in preterm birth rates remain. In 2022, the rate of preterm birth among African-American women (14.6%) was about 50 percent higher than the rate of preterm birth among white or Hispanic women (9.4% and 10.1% respectively).

A developing baby goes through important growth throughout pregnancy─ including in the final months and weeks. For example, the brain, lungs, and liver need the final weeks of pregnancy to fully develop. Babies born too early (especially before 32 weeks) have higher rates of death and disability. In 2021, preterm birth and low birth weight accounted for about 14.8% of infant deaths (deaths before 1 year of age). Babies who survive may have

• Breathing problems
• Feeding difficulties
• Cerebral palsy
• Developmental delay
• Vision problems
• Hearing problems

Preterm births may also take an emotional toll and be a financial burden for families.

Frequently Asked Questions: For Women and Their Families

What are the warning signs of preterm labor?

In most cases, preterm labor (labor that happens too soon, before 37 weeks of pregnancy) begins unexpectedly and the cause is unknown. Like regular labor, signs of early labor are—

• Contractions (the abdomen tightens like a fist) every 10 minutes or more often
• Change in vaginal discharge (a significant increase in the amount of discharge or leaking fluid or bleeding from the vagina)
• Pelvic pressure—the feeling that the baby is pushing down
• Low, dull backache
• Cramps that feel like a menstrual period
• Abdominal cramps with or without diarrhea

What should I do if I think I’m experiencing preterm labor?

If you think you are experiencing preterm labor, it is important that you see a healthcare provider right away. If you are having preterm labor, your provider may be able to give you medicine so that the baby will be healthier at birth.

Are some women more likely than others to deliver early?

Many times we do not know what causes women to deliver their babies early. However, several known factors may increase the likelihood that a woman could deliver early. Based on CDC Grand Rounds: Public Health Strategies to Prevent Preterm Birth, below are social, personal, and economic groups; pregnancy and medical conditions; and behavioral factors of women with high risk of early delivery.

Social, Personal, and Economic Groups

• Teens and women who become pregnant over age 35
• Black, Native Hawaiian or Other Pacific Islander, and American Indian or Alaska Native women*
• Women with lower incomes

Pregnancy and Medical Conditions

• Women who have had a prior preterm birth
• Women with an infection, including COVID-19
• Women carrying more than 1 baby (twins, triplets, or more)

Behavioral

• Women who use tobacco
• Women who use drugs
• Women experiencing stress

*Of note regarding racism and health, racism—both interpersonal and structural—negatively affects the mental and physical health of millions of people. The impact is pervasive and deeply embedded in our society—affecting where one lives, learns, works, worships and plays and creating inequities in access to a range of social and economic benefits—such as housing, education, wealth, and employment. These conditions—often referred to as social determinants of health—are key drivers of health inequities within communities of color, placing those within these populations at greater risk for poor health outcomes, including preterm birth.

Talk with your doctor about signs of preterm labor and what to do.

Can anything be done to prevent a preterm birth?

Preventing preterm birth remains a challenge because there are many causes of preterm birth, and because causes may be 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. These steps are to—

• Quit smoking. For help quitting, see How to Quit Smoking
• Avoid alcohol and 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
• Talk with your doctor or other healthcare provider about what interventions and monitoring are available if you had a previous preterm birth

Another step women and their partners can take to reduce the risk of preterm birth is waiting at least 18 months between pregnancies.

I used in vitro fertilization (IVF) or another assisted reproductive technology (ART) to get pregnant. What is my risk of having a preterm birth?

Women who conceive through ART are at higher risk for preterm birth, primarily because they are more likely to be pregnant with more than one baby at a time.

Is it okay to schedule my baby’s birth before 39 weeks of pregnancy?

Important growth and development occur throughout pregnancy—all the way through the final few weeks. Unless there is a medical need, delivery should not be scheduled before 39 weeks of pregnancy.

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.
• 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.
• Women with certain abnormalities of the reproductive organs are at greater risk for preterm labor and birth than are women who do not have these abnormalities. 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 include:

• Urinary tract infections
• Sexually transmitted infections
• Certain vaginal infections, such as bacterial vaginosis and trichomoniasis
• High blood pressure
• Bleeding from the vagina
• Certain developmental abnormalities 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.
• 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.
• Certain lifestyle and environmental factors, including:
• 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

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.
• 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.

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

• 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.

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 below.

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.

"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.

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 (pronounced sair-KLAZH). 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.

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.

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. 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.

Preterm birth can happen to anyone and many women who have a premature birth have no known risk factors. There are things, however, a woman can do to help your own health and lower the risk of having a premature baby.

• Quit smoking and avoid substances, such as alcohol or drugs.
• See your health care provider for a medical checkup before pregnancy.
• Work with your health care provider to control diseases, such as high blood pressure or diabetes.
• Get prenatal care early, as soon as you think you may be pregnant, and throughout the pregnancy.
• Discuss concerns during pregnancy with your health care provider, and seek medical attention for any warning signs or symptoms of preterm labor.

Birth is a complex but, fortunately, the outcome for most women is a full-term, healthy baby. More research still is needed to understand the risk factors for premature birth, such as how family history, genetics, lifestyle and environment may interact to put some women at greater risk for a premature delivery.