A preterm birth is any birth that happens before 37 weeks of pregnancy. A full-term pregnancy lasts about 40 weeks, so preterm (also called premature) babies arrive at least three weeks early. This affects roughly 1 in 10 pregnancies in the United States, making it one of the most common pregnancy complications. How early a baby arrives matters enormously: a baby born at 36 weeks faces very different challenges than one born at 28 weeks.
How Preterm Birth Is Classified
Not all preterm births carry the same level of risk. Doctors categorize them by how early the baby arrives:
- Late preterm: 34 to 36 weeks. This is the most common category, accounting for nearly 75% of all preterm births. These babies often do well but may still need extra time in the hospital.
- Moderately preterm: 32 to 33 weeks. Babies typically need several weeks of medical support as their lungs and other organs finish developing.
- Very preterm: 28 to 31 weeks. These babies require intensive care and face a higher chance of long-term health effects.
- Extremely preterm: Before 28 weeks. This is the most serious category. Survival rates have improved dramatically with modern medicine, but these babies face the greatest risks of complications.
The distinction matters because every additional week in the womb gives a baby’s organs, especially the lungs and brain, more time to mature. Even a difference of one or two weeks can meaningfully change outcomes.
What Causes Preterm Birth
In many cases, there’s no single identifiable cause. About half of all preterm births happen spontaneously, meaning labor starts on its own without a clear trigger. The other cases are “medically indicated,” where a doctor decides early delivery is safer for the mother, the baby, or both because of complications like preeclampsia or problems with the placenta.
For spontaneous preterm births, several factors increase risk. Infection is one of the most well-understood triggers. Infections in the uterus, cervix, or urinary tract can cause inflammation that stimulates contractions and weakens the membranes surrounding the baby. When those membranes rupture early (your water breaks before 37 weeks), preterm labor often follows.
A short or weakened cervix is another significant factor. The cervix normally stays closed and firm throughout most of pregnancy, then softens and opens as the due date approaches. In some women, this happens too early. Cervical length can be measured by ultrasound during pregnancy, and a measurement under 25 millimeters before 24 weeks is a red flag.
Risk Factors You Should Know
Some risk factors for preterm birth are things you can influence, while others are not. The strongest single predictor is having had a previous preterm birth. If your first baby came early, your risk of it happening again roughly doubles. Carrying twins or triplets also raises the risk substantially, with about 60% of twin pregnancies delivering before 37 weeks.
Other risk factors include:
- Smoking during pregnancy: Increases risk by roughly 20-30%
- Short interval between pregnancies: Getting pregnant within six months of a previous delivery raises risk
- Chronic health conditions: High blood pressure, diabetes, and autoimmune disorders all contribute
- Age: Both teenagers and women over 35 face higher rates
- Stress and socioeconomic factors: Chronic stress, lack of prenatal care, and limited access to nutrition all play a role
In the United States, there are stark racial disparities. Black women experience preterm birth at a rate of about 14%, compared to roughly 9% for white women. Research suggests this gap is driven not by genetics but by systemic factors including chronic stress from racism, unequal access to healthcare, and higher rates of underlying conditions linked to social inequality.
Warning Signs of Preterm Labor
Preterm labor doesn’t always announce itself dramatically. Some women experience obvious contractions, but others notice subtler changes. Recognizing the signs early can make a real difference, because treatments to delay delivery work best when started quickly.
The signs to watch for include regular contractions (a tightening feeling in your abdomen) that happen every 10 minutes or more often, pressure in your pelvis that feels like the baby is pushing down, a low dull backache that doesn’t go away when you change positions, menstrual-like cramping, and any change in vaginal discharge, especially if it becomes watery or tinged with blood. If you’re before 37 weeks and experience any of these, getting evaluated promptly matters. Doctors can check whether your cervix is changing and run a test on vaginal fluid that helps predict whether delivery is likely within the next two weeks.
How Doctors Manage Early Labor
If preterm labor is caught early enough, the goal is usually to delay delivery. Even buying 48 hours can be significant, because that window allows doctors to give corticosteroid injections that help the baby’s lungs mature faster. This single intervention has been one of the biggest advances in premature baby survival. Babies who receive this benefit before delivery have lower rates of breathing problems and other complications.
Medications can temporarily slow or stop contractions, though they typically buy days rather than weeks. If delivery seems inevitable before 32 weeks, a medication called magnesium sulfate is often given to help protect the baby’s developing brain.
For women identified as high risk before labor starts, there are preventive options. Progesterone supplementation, given as weekly injections or a daily vaginal gel, can reduce the chance of recurrent preterm birth in women who’ve delivered early before. A cervical cerclage, a stitch placed around the cervix to help keep it closed, is another option for women with a short cervix or history of cervical insufficiency.
What Preterm Birth Means for the Baby
The most immediate concern for preterm babies is their lungs. The lungs are one of the last organs to fully develop, and babies born early often can’t breathe well on their own. Depending on how premature they are, they may need supplemental oxygen, a breathing machine, or a substance called surfactant that helps keep the tiny air sacs in their lungs open.
Beyond breathing, preterm babies may struggle to maintain their body temperature, feed effectively, and fight off infections. Their digestive systems are immature, which can lead to a serious intestinal condition. Their brains are still developing rapidly, making them vulnerable to bleeding and injury. The earlier the birth, the higher the risk for each of these issues.
The good news is that outcomes have improved enormously over the past few decades. Babies born at 28 weeks now have survival rates above 90% in countries with advanced neonatal care. Even at 24 weeks, often considered the edge of viability, survival rates have climbed to around 40-70% depending on the hospital. Late preterm babies (34-36 weeks) generally do very well, though they may spend extra days in the hospital and have slightly higher rates of early health issues compared to full-term babies.
Long-Term Effects
Most late preterm babies catch up to their full-term peers developmentally within the first year or two. For babies born earlier, the picture is more variable. Some very preterm babies develop completely typically. Others may face challenges with learning, attention, motor coordination, or vision and hearing.
Cerebral palsy is more common among very preterm babies, though the overall rate is still relatively low at around 5-10% of babies born before 28 weeks. Learning differences and attention difficulties show up more often as preterm children reach school age. These aren’t inevitable outcomes, and early intervention programs can make a meaningful difference. Babies born prematurely are typically followed closely by developmental specialists throughout early childhood to catch any delays as early as possible.
There’s also growing evidence that preterm birth has subtle effects that extend into adulthood. Adults who were born very premature have slightly higher rates of high blood pressure and metabolic issues. These effects are generally modest, but they underscore how the conditions of early life shape long-term health.