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Learn what causes preterm labor and what doctors can do to manage it

Updated on December 9, 2015.

By Elaine Brown, MD

What is preterm labor?

Preterm labor is labor that occurs after 20 weeks of pregnancy and before 37 completed weeks. Your due date marks 40 weeks of pregnancy. Your pregnancy is considered full-term once you mark 37 weeks from your last menstrual period.

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, hearing impairment and learning disabilities. Preterm labor and delivery can result in health problems for the mother, too, along with high healthcare costs. In the United States, 12% of pregnancies end in preterm birth.

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%

Only your healthcare provider can confirm the last two, but you should always check with them about vaginal bleeding, especially if it’s in conjunction with contractions.

Which tests determine whether early contractions are “false labor” or true preterm labor?

Unless you’re in active labor with advanced cervical dilation, preterm labor can be difficult to diagnose. About 50% of women suspected of having preterm labor go on to deliver full term. To find out if you’re at risk for preterm delivery or just having contractions, you’ll undergo some testing:

• Cervical exam. Your provider will check to see if your cervix is dilated. If dilation is more than 3 cm or the cervix is very thin, you’ll likely be admitted with the diagnosis of preterm labor. If your cervix is still longer than 3 cm (meaning it’s barely dilated), this effectively rules out preterm labor.
 
• Transvaginal ultrasound. If your cervix is dilated to less than 2 cm, according to a cervical exam, you may have a transvaginal ultrasound (in which the ultrasound probe is inserted into your vagina rather than applied to the belly) to further check it. If your cervix is less than 1.5 cm long (meaning you’re significantly dilated), you’ll be treated for preterm labor. If your cervical length is in between, a fetal fibronectin test may be recommended.
 
• Fetal fibronectin test. Fibronectin is a protein that’s been associated with preterm labor. If the test is negative (meaning no fibronectin was detected), a doctor can send the patient home with the confidence that she’s unlikely to deliver in the next few days.

What happens if preterm labor is diagnosed?

If it appears that delivery may be imminent, a provider typically administers magnesium sulfate to the mom through an IV or intravenous line. This medication is given in the attempt to stop preterm labor long enough to transfer her to a major or specialty hospital with a neonatal intensive care unit (NICU) capable of caring for premature infants. Additionally, recent studies have shown that magnesium sulfate has a protective effect for babies, 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 given to the mom. Steroids help the fetal lungs mature more quickly, reducing the newborn's need for oxygen. Oxygen delivered in high concentrations for prolonged periods is associated with vision problems and hearing impairment.

Other drugs that decrease contractions (known as tocolytics) may also be given in an attempt to slow down labor. Antibiotics may be administered to protect the baby, as well. 

While all this is going on, a provider will also attempt to find the cause behind the preterm labor. This can include infection, an elevated level of amniotic fluid, premature rupture of the amniotic sac, or placental abruption (when the placenta releases from the uterine wall before delivery), as well as a pregnancy complication known as preeclampsia, which is marked by high blood pressure in the mom-to-be. Certain complications are treated with early delivery of the baby, so preterm labor might be allowed to continue in those cases.

Can preterm labor be predicted and prevented before it begins?

It’s difficult to predict who will have preterm labor.   

When it comes to prevention, 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 to close it) has also been demonstrated to reduce the number of preterm births, in certain cases. 

Who is at risk for preterm delivery?

You’re at the highest risk for preterm delivery if you:

• have a history of premature delivery in a previous pregnancy 
• are carrying twins or higher-order multiples (triplets or more); 61% of multiple pregnancies deliver before 37 weeks
• got pregnant soon after delivering a child (known as a “short pregnancy interval”)
• have a short cervical length
• have undergone treatment for cervical dysplasia, such as a LEEP procedure, which means you are more likely to have a short cervix
• had a low pre-pregnancy weight
• smoke or abuse other substances
• have had certain infections
• have experienced vaginal bleeding during pregnancy.

Many of these factors aren’t in your control, but substance abuse and smoking are; quitting is important for a healthy pregnancy and safe delivery of your baby. 

Published on March 6, 2014.

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