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Preterm Labor: Who’s at Risk and How to Prevent It


Learn what causes preterm labor, who’s at risk, and what doctors can do to manage it

By Elaine Brown, MD

What is preterm labor?

Preterm labor is labor on or before 37 completed weeks of pregnancy (normally pregnancy is 40 weeks long).

Why is preterm labor undesirable?

Infants born prematurely (especially before 32 weeks) are more likely to suffer complications such as cerebral palsy, chronic lung disease, visual disturbances, and hearing impairment.

What are the symptoms of preterm labor?

Symptoms of preterm labor include:

  • More than 4 to 6 contractions per hour for several hours in a row
  • Vaginal bleeding
  • Ruptured membranes ("broken water")
  • Cervical dilation of more than 3 cm or effacement (thinning) of greater than 80%


What causes preterm labor?

There are four major pathways that lead to preterm birth:

  1. The strongest risk factor for preterm labor is a history of a previous preterm birth.
  2. Multiples pregnancy (carrying twins or more).
  3. Vaginal bleeding or blood seeping between the placenta and uterine wall.
  4. Premature activation of the signals that lead to term labor; these are signals are still being discovered.

What tests are used to determine whether early contractions are "false labor" or true preterm labor?

Many doctors give their patients a single injection of terbutaline when she presents to the hospital with contractions. If labor stops completely with one injection, it was probably a false alarm. If contractions continue, additional diagnostic tests may be used to help sort out true from false pre-term labor. Some of these are:

  • Transvaginal ultrasound: cervical length of greater than 3 cm effectively rules out preterm labor, while a cervix which is shorter than 2 cm is concerning.
  • Fetal fibronectin test: Fibronectin is a protein that glues the membranes to the uterine wall; fibronectin is detected by swabbing the vaginal walls of the expectant mother. Fibronectin is not present unless the membranes are loosening. If the test is negative (meaning no fibronectin was detected), a doctor can send the patient home with confidence that she is unlikely to deliver in the next few days.


What is done if true preterm labor is diagnosed?

If it appears that delivery may be imminent, magnesium sulfate is typically administered to the mother. Magnesium sulfate is given by IV in the attempt to stop true preterm labor long enough to transfer the mother to a tertiary care hospital (a major or specialty hospital) with a neonatal intensive care unit (NICU) that is capable of caring for premature infants. Recent studies have indicated that magnesium sulfate, in addition to slowing labor, has a protective effect of stabilizing delicate fetal blood vessels, especially those in the brain. When administered to mothers who deliver prematurely, it reduces the risk of cerebral palsy (a neurologic disorder) and necrotizing enterocolitis (a severe infection of the bowel) in their infants.

During this time period, corticosteroids are often also administered. Corticosteroids, such as beta methasone, help the fetal lungs to mature quickly, reducing the newborn's need for oxygen. Oxygen delivered in high concentrations for prolonged periods of time is associated with vision problems and hearing impairment.


Who is at highest risk for preterm delivery?

The following groups of women are at highest risk for preterm delivery:

  • Mothers with a history of premature delivery in a previous pregnancy are at highest risk.
  • Mothers with twins or higher order multiples (triplets or more); 61% of multiple pregnancies deliver before 37 weeks.
  • Mothers with bacterial vaginosis have traditionally been thought to have an increased risk, but recent studies have had conflicting results, suggesting that this may not be a risk factor after all.
  • Mothers with a short cervical length. Measurement of the cervix is now a standard part of a mid-pregnancy ultrasound; mothers with a cervical length of less than 2 cm measured by transvaginal ultrasound are at significant risk. Mothers who have undergone treatment for cervical dysplasia such as conization or LEEP procedures are more likely to have a short cervix.

Finally, other factors associated with preterm delivery are vaginal bleeding, urinary tract infection during the current pregnancy, sexually transmitted infections, smoking and periodontal disease.


Can preterm labor be predicted and prevented before it begins?

Recent studies have shown that administration of 17 alpha hydroxyprogesterone caproate (typically given to mothers with a history of preterm birth in a previous pregnancy) statistically reduces preterm deliveries.

Cervical cerclage (a suture similar to a purse string which is placed around the cervix) has also been demonstrated to reduce the number of preterm births. A cerclage can be used for women who have a history of preterm delivery and who have not responded to progesterone therapy.

The incidence of preterm birth in the United States increased by 20% between 1990 and 2006. Although much of this increase was due to an increase in multiple gestations resulting from assisted reproductive technologies (such as IVF) — multiple pregnancies increased by 22% over approximately the same time period — newer and more effective strategies are needed to address preterm labor and birth.

Current research is increasing our understanding of the mechanisms that cause preterm birth. As this knowledge increases, our ability to diagnose, treat and prevent preterm birth should improve substantially. There is good news too for those who go on to deliver early despite intervention: Our ability to care for premature infants, even extremely tiny babies (22-23 weeks), has resulted in increased survival of preterm infants with fewer disabilities.


Dr. Elaine Brown completed her residency in obstetrics and gynecology at Harvard. She has more than 15 years of experience in private practice.


Published March 6, 2014 


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Reviewed by Elaine Brown, MD on February 10, 2014