Preterm delivery is any birth that happens before 37 completed weeks of pregnancy. A full-term pregnancy lasts about 40 weeks, so babies born even a few weeks early fall into this category. In the United States, about 1 in 10 babies arrives preterm, with the rate holding steady at 10.41% in 2024.
Not all preterm births carry the same risks. The earlier a baby is born, the more likely they are to face serious health challenges, both immediately and later in life. Understanding the categories, causes, and warning signs can help you recognize what’s happening and what to expect.
Categories of Preterm Birth
Doctors divide preterm birth into four groups based on how early the baby arrives:
- Late preterm: born between 34 and 36 completed weeks. This is the most common type and generally carries the lowest risk, though these babies still face more complications than full-term infants.
- Moderately preterm: born between 32 and 34 weeks.
- Very preterm: born between 28 and 32 weeks. These babies typically need extended time in a neonatal intensive care unit.
- Extremely preterm: born before 28 weeks. This group faces the greatest challenges, with about 15% of extremely preterm survivors developing cerebral palsy.
What Causes Preterm Delivery
Preterm births fall into two broad categories: spontaneous and medically indicated. Spontaneous preterm births, which account for roughly 65 to 70% of all preterm deliveries, happen either when labor starts on its own before 37 weeks or when the membranes surrounding the baby rupture early. About 40 to 45% of preterm births follow spontaneous labor with intact membranes, and another 25 to 30% follow early membrane rupture.
The remaining 30 to 35% are medically indicated, meaning a doctor recommends early delivery because continuing the pregnancy poses serious risks to the mother or baby. Common reasons include preeclampsia (dangerously high blood pressure during pregnancy), restricted fetal growth, placental problems causing vaginal bleeding, and complications from conditions like diabetes or thyroid disease.
Spontaneous preterm labor doesn’t always have a single identifiable cause. Infection or inflammation in the uterus, problems with blood flow to the placenta, and overdistension of the uterus (as with twins or excess amniotic fluid) all contribute. Carrying multiples raises the risk significantly: about 40% of twin pregnancies result in spontaneous labor or membrane rupture before 37 weeks.
Warning Signs of Preterm Labor
Preterm labor can develop gradually or come on suddenly. The key signs to watch for include:
- Regular contractions: a tightening sensation in your abdomen that comes every 10 minutes or more often. Six or more contractions in a single hour is not normal at any point before full term.
- Changes in vaginal discharge: a sudden increase in discharge, leaking fluid, or any vaginal bleeding.
- Pelvic pressure: a persistent feeling of heaviness or pressure low in the pelvis.
- Low backache: a dull, constant ache in the lower back that doesn’t go away with position changes.
- Menstrual-like cramping: cramping in the lower abdomen, sometimes accompanied by diarrhea.
These symptoms can be subtle and easy to dismiss, especially in a first pregnancy where you may not know what contractions feel like. If you notice any combination of these before 37 weeks, getting evaluated quickly matters because early intervention can sometimes delay delivery long enough to make a significant difference for the baby.
How Doctors Predict and Diagnose It
When a pregnant person arrives with symptoms of preterm labor, doctors use two main tools to assess whether delivery is truly imminent. The first is a transvaginal ultrasound to measure cervical length. A normal cervix during pregnancy is typically 30 mm or longer. A cervix that has shortened to 25 mm or less signals a significantly higher risk, and measurements of 20 mm or below are particularly concerning, with a very high specificity for predicting preterm birth.
The second tool is a fetal fibronectin test, which checks for a protein in vaginal secretions. This protein acts as a kind of biological glue between the amniotic sac and the uterine lining, and elevated levels (50 ng/mL or higher after 22 weeks) suggest that the connection is breaking down. The test is most useful for its negative predictive value: a negative result is reassuring that delivery is unlikely within the next couple of weeks.
For women with a history of preterm birth before 34 weeks, serial cervical length measurements from 16 to 24 weeks are recommended to catch shortening early, when preventive treatment can still help.
Prevention Strategies
Two proven interventions can reduce the risk of preterm delivery in specific situations. Vaginal progesterone, a hormone supplement inserted daily, is effective for women found to have a short cervix (10 to 20 mm) during a midpregnancy ultrasound, regardless of whether they’ve had a prior preterm birth. It reduces the chance of delivering before 33 weeks and also lowers the rate of serious complications for the newborn. Universal cervical screening in the second trimester, followed by progesterone for those with short measurements, appears to be a cost-effective approach that catches at-risk pregnancies in women who have no prior history of early delivery.
Cervical cerclage, a procedure in which a stitch is placed around the cervix to help keep it closed, is an option for women who have both a short cervix (under 25 mm before 24 weeks) and a prior spontaneous preterm birth. This combination of risk factors identifies the women most likely to benefit from the procedure.
What Happens When Preterm Delivery Can’t Be Prevented
When preterm delivery appears unavoidable, the priority shifts to giving the baby the best possible start. If there’s time, steroid medications given to the mother can accelerate the baby’s lung development. These are most effective when given between 24 and 34 weeks of pregnancy, at least 24 to 48 hours before delivery. For late preterm pregnancies (34 to 36 weeks), steroids may also be beneficial if the mother hasn’t received them previously. These medications meaningfully reduce the risk of breathing problems, bleeding in the brain, and other serious newborn complications.
For very early deliveries (before 32 weeks), treatment to protect the baby’s brain development may also be administered before birth. Both of these interventions work best with some lead time, which is one reason why recognizing preterm labor symptoms early matters so much.
Survival by Gestational Age
Survival rates for preterm babies have improved dramatically with advances in neonatal care, but gestational age remains the single strongest predictor of outcomes. At 22 weeks, survival is possible but uncommon, with rates ranging from about 3 to 17% depending on the hospital and level of intervention. At 23 weeks, survival rises to roughly 25 to 38%. By 24 weeks, about half to 60% of babies survive, and at 25 weeks, survival reaches approximately 73 to 79%. After 28 weeks, the vast majority of preterm infants survive with appropriate medical care, though they may still face a lengthy hospital stay.
Each additional day and week in the womb matters enormously at these early gestational ages. Even delaying delivery by 48 hours can allow time for lung-maturing treatments that improve a baby’s chances.
Long-Term Health Effects
Many preterm babies grow up healthy, but prematurity can leave lasting traces on several body systems. The lungs are particularly affected. Adults who were born preterm have measurably lower lung function than those born at term, with lung capacity reduced by about 7% on average. For those who developed chronic lung disease in the newborn period, the reduction can reach 17 to 19%. This translates to more respiratory symptoms in everyday life, higher rates of asthma-like breathing problems, and roughly a 40% higher chance of sleep-disordered breathing (like snoring and sleep apnea) in early adulthood.
Neurological and developmental effects are also well documented. Preterm birth is the most commonly identified cause of cerebral palsy, and cognitive challenges, including difficulties with attention, processing speed, and executive function, are more common among preterm survivors. The risk is highest for the most premature babies, but even late preterm infants show slightly elevated rates of learning difficulties compared to their full-term peers.
Cardiovascular and metabolic risks also track with prematurity into adulthood, including higher rates of high blood pressure and diabetes. These associations hold across gestational ages, with each additional week of pregnancy providing a small but measurable protective effect that persists through at least the mid-40s.