The term (or length of time) of a normal human pregnancy is about 40 weeks (9.2 months). Labor that begins before 37 weeks of pregnancy is called preterm labor (or early labor). An infant born before 37 weeks of development in the uterus is considered a preterm birth.

Late-preterm birth refers to infants born from 34 weeks through 36 weeks of pregnancy. More than 70% of preterm infants are born during this time.

Many organs, including the brain, lungs, and liver, need the final weeks of pregnancy to fully develop. Even infants born slightly preterm are at greater risk than full-term infants, but the earlier the delivery, the more likely the risk of serious disability or death. Adverse health outcomes related to preterm birth include 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. Preterm births also cause heavy emotional and economic burdens for families

Symptoms of preterm labor

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.

Risk Factors for preterm labor and birth

Any pregnant woman could have preterm labor and birth, but there are some factors that increase a woman's risk of going into labor before 37 weeks of pregnancy. One of the most important risk factors for delivering preterm is a history of a previous infant born preterm.

  • Being pregnant with more than one fetus at a time (twins or triplets or more) 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 (uterus and cervix) are at greater risk of going into labor early.

Certain medical conditions during pregnancy increase the likelihood that a woman will have preterm labor. These conditions include:

  • Having a short cervix in the second trimester (fourth through sixth months) of pregnancy
  • Urinary tract infections
  • Sexually transmitted infections
  • Certain vaginal infections, such as bacterial vaginosis and trichomoniasis
  • Diabetes (high blood sugar)
  • High blood pressure
  • Blood clotting problems
  • Bleeding from the vagina
  • Certain developmental abnormalities in the fetus
  • Pregnancy resulting from in vitro (in a living body) fertilization
  • Being underweight before pregnancy
  • Obesity (although some studies show that obesity actually protects against preterm birth)
  • Short time period between pregnancies (less than 6 months between a birth and the beginning of the next pregnancy)

Shortened Cervix and Preterm Birth

As a preparation for birth, the cervix (the lower part of the uterus) naturally shortens late in pregnancy. However, having a prematurely shortened cervix, around the fourth or fifth month of pregnancy, increases the risk of delivering preterm.

An ultrasound can be done to evaluate a woman's cervix during pregnancy. Transvaginal ultrasound scans to measure cervical length can identify women with a shortened cervix.

"Incompetent" Cervix and Preterm Birth

Doctors may refer to a cervix that begins to open before an infant is ready to be born as an "incompetent" cervix. The process of cervical opening is painless and unnoticeable, without labor contractions or cramping. The cervix (the neck of the uterus) normally stays tightly closed during pregnancy. A cervix that starts to open early can lead to miscarriage. For some women, this happens again in later pregnancies. This may be due to cervical weakness (incompetence) if the miscarriage occurred in the second or early third trimester.

What is meant is that the cervix is shortened, but some women may have abnormalities of the cervix that make it harder for the cervix to remain closed during pregnancy.

To try to prevent preterm labor and birth, a doctor may place a stitch around the cervix to keep it closed. This procedure is called cervical cerclage. For women with a prior preterm birth who have a short cervix, cerclage may improve their prospects for normal birth.

Fetal Fbronectin (fFN) Test

The fetal fibronectin test is used to detect whether the protein fFN is being produced. Fetal fibronectin is like a biological "glue" between the uterine lining and the pregnancy.

Normally fFN can be detected in the pregnant woman's secretions from the vagina and cervix early in the pregnancy (up to 22 weeks, or about 5 months) of pregnancy and again toward the end of the pregnancy (1 to 3 weeks before labor). 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.

Testing for fFN can predict with about 50% accuracy which pregnant women showing signs of preterm labor are likely to have a preterm birth. 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.

Diagnosis of preterm labor

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 get checked. 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.

The health care provider will perform a pelvic exam to see if the membranes have ruptured or if the cervix is beginning to get thinner (efface) or open (dilate). An ultrasound exam may be done, and a monitor may be used to electronically record contractions and the fetal heart rate.

Prevention of Preterm Labor and birth

Hormone treatment. The only preventive drug therapy is progesterone, 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. This preventive therapy is given beginning at 16 weeks of gestation and continues to 37 weeks of gestation. The treatment works among all ethnic groups and can improve outcomes for infants.

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). The term "incompetent cervix" often refers to a shortened cervix diagnosed by a cervical ultrasound. In the cerclage procedure, a doctor stitches the cervix closed. The stitch is then removed closer to the woman's due date.

Treatments of Preterm Labor and Birth

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 - Tocolytics

Drugs called tocolytics 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, 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 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. Intravenously delivered magnesium sulfate may also reduce the risk of cerebral palsy if the child is born early.

Effects of Preterm Birth on Families

Preterm birth can have profound effects on families, both emotionally and financially. In addition to the time spent in the NICU, preterm infants spend an average of 17 days in the hospital, compared to just over 2 days for full-term infants; they also have 50% more doctor's office visits in the first year.

In the first 6 months of the preterm infant's life, the mother will miss, on average, almost 6 weeks of work—2 weeks more than most mothers of full-term infants. 

Having a preterm infant can cause hardships and emotional challenges for families. 

Reference: National Institutes of Child Health and Human Development

This information is for general educational uses only. It may not apply to you and your personal medical needs. This information should not be used in place of a visit, call, consultation with or the advice of your physician or health care professional.

Communicate promptly with your physician or other health care professional with any health-related questions or concerns.

Be sure to follow specific instructions given to you by your physician or health care professional.

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