Merck Manual

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Preterm Labor


Julie S. Moldenhauer

, MD, Children's Hospital of Philadelphia

Reviewed/Revised Jul 2021 | Modified Sep 2022

Labor that occurs before 37 weeks of pregnancy is considered preterm.

  • Babies born prematurely can have serious health problems.

  • The diagnosis of preterm labor is usually obvious.

  • Measures such as rest and sometimes drugs may be used to delay labor.

  • Antibiotics or corticosteroids may also be needed.

What causes preterm labor is not well understood. However, certain conditions may make it more likely:

A healthy lifestyle during pregnancy can help reduce the risk of preterm labor, as can regular visits to the doctor or midwife, who can then identify potential problems early.

Diagnosis of Preterm Labor

  • A doctor's evaluation

The diagnosis of preterm labor is usually obvious, based on the signs that labor is starting and the length of the pregnancy.

Samples may be taken from the cervix, vagina, and anus to culture. Analysis of these samples may suggest a specific infection as the cause of preterm labor.

A sample of urine may be analyzed and cultured (placed in conditions that encourage the growth of microorganisms) to check for kidney and bladder infections.

Treatment of Preterm Labor

  • Sometimes allowing labor to continue

  • Antibiotics until infection is ruled out

  • If labor needs to be delayed, rest, fluids, drugs that slow labor, antibiotics, and sometimes corticosteroids

If vaginal bleeding occurs or the membranes around the fetus rupture, allowing labor to continue is often best.

If vaginal bleeding does not occur and the membranes are not leaking amniotic fluid (the fluid that surrounds the fetus in the uterus), the woman is advised to rest and to limit her activities as much as possible, preferably to sedentary ones. She is given fluids and may be given drugs that can slow labor. These measures can often delay labor for a brief time.

Drugs that can slow labor include the following:

  • Magnesium sulfate: This drug is often given intravenously to stop preterm labor, particularly if the pregnancy is less than 32 weeks. Magnesium sulfate also appears to substantially reduce the risk of bleeding in the brain and the resulting problems with development of the newborn's brain, such as cerebral palsy. However, if the dose is too high, it may affect the woman’s heart and breathing rates.

  • Calcium channel blockers: These drugs are usually used to treat high blood pressure. They sometimes cause headaches and low blood pressure in the woman.

  • Prostaglandin inhibitors: These drugs may temporarily reduce the amount of amniotic fluid. They are not used after the 32nd week of pregnancy because they may cause heart problems in the fetus.

Women are given antibiotics until culture results are obtained. If results are negative, the antibiotics are then stopped.

If the cervix opens (dilates) more than 2 inches (5 centimeters), labor usually continues until the baby is born.

If the membranes rupture between the 24th and 34th week of pregnancy, corticosteroids are given to help the fetus’s lungs mature, unless delivery is expected soon. Doctors also consider giving women corticosteroids if the membranes rupture

  • Between 34 and 37 weeks of pregnancy if women are at risk of preterm delivery and have not been given any corticosteroids earlier in the pregnancy

  • Starting at 23 weeks of pregnancy if women are at risk of preterm delivery within 7 days

The corticosteroid helps the fetus’s lungs and other organs mature more quickly. It also reduces the risk that after birth, the baby will have difficulty breathing (neonatal respiratory distress syndrome Respiratory Distress Syndrome in Newborns Respiratory distress syndrome is a lung disorder in premature newborns in which the air sacs in their lungs do not remain open because a substance that coats the air sacs called surfactant is... read more ) or other problems related to prematurity (such as bleeding in the brain).

In future pregnancies, women who have had a preterm delivery may be given a progestin. This drug may reduce the risk of having another preterm delivery. The progestin is started during the 2nd trimester and continued until just before delivery.

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