About 1 in 10 babies in the US is born before 37 weeks, making the country an outlier among high-income nations for preterm birth rates. Preterm labor is the entry point for most early births, and recognizing it allows for interventions that can either delay delivery or, when delivery is unavoidable, prepare the baby for life outside the uterus. The mechanisms behind preterm labor are still incompletely understood, but the practical steps to identify and manage it are well established.
This guide covers warning signs, established and evolving risk factors, current treatment approaches, and what early birth means for both short-term newborn outcomes and long-term child health. For closely related conditions, see our guides on preeclampsia and group B strep, both within our medical conditions library.
What Preterm Labor Is
Preterm labor is regular uterine contractions accompanied by cervical change (dilation, effacement, or both) before 37 weeks of pregnancy. Preterm birth is delivery before 37 weeks. The categories matter clinically: extremely preterm (before 28 weeks), very preterm (28 to 32 weeks), moderate-to-late preterm (32 to 37 weeks). About 70 percent of preterm births are late preterm.
Some preterm births are spontaneous (resulting from labor or premature membrane rupture); others are medically indicated for conditions like severe preeclampsia, intrauterine growth restriction, or placental abruption. Each pathway has different prevention strategies.
Warning Signs to Recognize
Symptoms of preterm labor can be subtle and easy to dismiss as normal pregnancy discomfort. Common signs include regular contractions (more than four to six per hour, often felt as tightening of the abdomen), low backache that comes and goes, pelvic pressure or a feeling that the baby is pushing down, menstrual-like cramping, change in vaginal discharge (especially watery, mucus-like, or bloody), and fluid leaking from the vagina (rupture of membranes).
Braxton-Hicks contractions, which are normal, are typically irregular, painless, and resolve with hydration or rest. Persistent regular contractions every 10 minutes or less, particularly with cervical pressure or back pain, deserve evaluation.
When to seek emergency care: Call 911 or go to the nearest labor and delivery triage if you have regular contractions before 37 weeks (especially every 10 minutes or less), fluid leaking from the vagina, vaginal bleeding, severe pelvic pressure, or a noticeable decrease in fetal movement. Early evaluation can preserve treatment options.
Risk Factors
The strongest single risk factor is a prior preterm birth, with recurrence rates of 15 to 30 percent depending on the gestational age of the prior delivery and underlying cause. Multiple gestation (twins, triplets) carries high risk; about 60 percent of twins and almost all triplets deliver preterm.
Other risk factors include short cervix on second-trimester ultrasound (less than 25 mm at 18 to 24 weeks), uterine anomalies, previous cervical surgery (LEEP, conization), prior cesarean, infections including bacterial vaginosis and untreated urinary tract infections, smoking, illicit drug use, periodontal disease, low pre-pregnancy BMI, advanced maternal age (over 35), short interpregnancy interval (less than 6 months), and pregnancy conceived with IVF.
Many preterm births occur in women without identifiable risk factors. Black women in the US experience preterm birth at rates roughly 50 percent higher than white women, a disparity reflecting structural healthcare inequities, chronic stress exposure, and other factors that medical interventions alone cannot fully address.
Diagnosis
Evaluation typically includes assessment of contraction pattern (continuous monitoring for at least 1 to 2 hours), sterile speculum exam to check for membrane rupture and cervical dilation, transvaginal ultrasound to measure cervical length, and tests for infection. Fetal fibronectin (fFN), a glycoprotein, is sometimes tested in symptomatic women between 22 and 34 weeks; a negative result has high negative predictive value, meaning preterm birth is unlikely in the next 1 to 2 weeks. A positive fFN is less specific and does not necessarily mean labor is imminent.
Cervical length less than 25 mm by transvaginal ultrasound between 18 and 24 weeks identifies women at higher risk in singleton pregnancies. Some practices screen all women; others screen only those with risk factors.
Treatment to Delay Delivery
When preterm labor is confirmed, treatment goals are to delay delivery long enough to administer corticosteroids (which mature the baby’s lungs), give magnesium sulfate (which reduces cerebral palsy risk in extreme prematurity), administer GBS prophylaxis if warranted, and arrange transfer to a hospital with appropriate neonatal care if needed.
Corticosteroids. Two doses of betamethasone given 24 hours apart, or four doses of dexamethasone given 12 hours apart, between 24 and 34 weeks. Corticosteroids reduce neonatal respiratory distress syndrome, intraventricular hemorrhage, and mortality, per the ACOG Committee Opinion 825. They are also considered between 22 and 24 weeks if delivery is imminent and the family chooses active intervention, and selectively in late preterm (34 to 36 weeks) for women at high risk of imminent delivery.
Magnesium sulfate. IV magnesium given within 24 hours before delivery before 32 weeks reduces cerebral palsy risk in surviving infants by about 30 percent.
Tocolytics. Medications that briefly delay delivery (typically 48 hours or less) include nifedipine (calcium channel blocker), indomethacin (NSAID, used before 32 weeks), and terbutaline (rarely used now due to maternal side effects). Tocolytics buy time for steroids to work; they do not improve long-term pregnancy outcomes by themselves.
Prevention Strategies
For women with a prior spontaneous preterm birth, options include vaginal progesterone, often started in the second trimester. Cervical cerclage (a stitch placed in the cervix) is offered to women with prior preterm birth and short cervix on ultrasound. Recent guidance from ACOG, following the PROLONG trial, no longer recommends 17-OHP (Makena) for routine prevention in all women with prior preterm birth.
For women without prior preterm birth but found to have a short cervix on routine ultrasound, vaginal progesterone reduces preterm birth risk. Cerclage is not generally recommended in this group without prior preterm birth.
Other measures with evidence include treating asymptomatic bacteriuria, treating bacterial vaginosis in some symptomatic women, smoking cessation, treating periodontal disease, and adequate prenatal care.
What Early Birth Means for the Baby
Outcomes vary dramatically by gestational age at birth. Term infants (37 weeks and beyond) typically do well. Late preterm infants (34 to 36 weeks) generally do well but have higher rates of feeding difficulties, jaundice, transient breathing problems, and NICU stays compared with term infants.
Moderately preterm (32 to 33 weeks) infants usually need NICU care for breathing support and feeding maturity. Very preterm (28 to 31 weeks) infants face higher risks of respiratory distress syndrome, intraventricular hemorrhage, necrotizing enterocolitis, and longer-term developmental concerns. Extremely preterm (before 28 weeks) is the highest-risk category, with survival rates rising steadily over recent decades but with substantial neurodevelopmental risks for survivors.
Modern neonatal intensive care has dramatically improved outcomes at every gestational age. Survival at 22 to 23 weeks now occurs in some centers, though with high rates of significant disability among survivors. Counseling at the threshold of viability involves family preferences, local outcomes, and individual baby factors.
Frequently Asked Questions
Can preterm labor be stopped?
Often only briefly. Tocolytic medications can delay delivery by 48 to 72 hours, which allows critical interventions like corticosteroids and magnesium sulfate. Long-term continuation of pregnancy after preterm labor is unusual; the most common course is delivery within 1 to 7 days of the labor episode.
What is the earliest a baby can survive?
Survival has been documented as early as 21 to 22 weeks in tertiary care centers, with rates improving steadily. Outcomes at the threshold of viability remain highly variable, and decisions about active intervention typically involve discussion between families and neonatology teams.
Can I prevent preterm labor in my next pregnancy?
Women with a prior preterm birth should be evaluated by a maternal-fetal medicine specialist. Vaginal progesterone, cervical length monitoring, and cerclage (when indicated) reduce recurrence. Lifestyle measures including not smoking, treating infections, and adequate prenatal care also help.
What does a short cervix mean?
A cervical length less than 25 mm on transvaginal ultrasound between 18 and 24 weeks indicates higher preterm birth risk. Vaginal progesterone reduces this risk. In women with prior preterm birth and short cervix, cerclage is also considered.
The Bottom Line
Preterm labor is one of the most consequential events in pregnancy, and recognizing it early opens up the most options. Knowing the warning signs, understanding personal risk based on history, and having a low threshold for triage evaluation all shape outcomes. Modern obstetrics has effective tools (corticosteroids, magnesium, targeted use of progesterone and cerclage) that meaningfully change neonatal outcomes when used at the right time.