Babies are born premature for a wide range of reasons, from infections and inflammation in the uterus to chronic health conditions in the mother to simply carrying more than one baby. An estimated 13.4 million babies were born preterm in 2020, more than 1 in 10 worldwide. Sometimes labor starts on its own too early, sometimes doctors make the call to deliver early because waiting would be dangerous, and sometimes the cause is never clearly identified.
How the Body Triggers Labor Too Early
Normal labor begins when a cascade of inflammation signals the uterus to shift from its quiet, pregnancy-maintaining state into active contractions. In a full-term pregnancy, this process is tightly regulated and happens around 39 to 40 weeks. In preterm labor, the same inflammatory switch gets flipped too soon.
The tissues connecting the mother and baby, including the lining of the uterus and the membranes surrounding the fetus, begin producing higher levels of hormone-like compounds called prostaglandins, along with stress hormones and inflammatory signals. These chemicals soften the cervix, break down the structural proteins holding the membranes intact, and stimulate contractions. When this process activates before 37 weeks, the result is spontaneous preterm labor. What makes preterm birth so difficult to predict is that different tissues and organ systems can independently set off this chain of events through different pathways.
Infection and Inflammation
Infection is one of the most well-understood triggers of premature labor. Bacteria that travel into the uterus, often from the vagina, can provoke a strong immune response in the surrounding tissues. The body essentially treats the infection as a reason to evacuate, ramping up the same inflammatory signals that drive labor. Urinary tract infections, bacterial vaginosis, and gum disease have all been linked to higher preterm birth risk, even when the infection is far from the uterus. Systemic inflammation from any source can contribute.
This is one reason prenatal care screens for infections. Treating bacterial vaginosis or urinary tract infections during pregnancy can reduce, though not eliminate, the chance of early labor.
Carrying Twins or Triplets
Multiple pregnancies are one of the strongest predictors of preterm birth. The average delivery for a single baby happens around 38.8 weeks. For twins, that drops to 35.3 weeks. Triplets average 32.2 weeks, and quadruplets just 29.9 weeks.
Compared to singleton pregnancies, multiples carry a nine-fold increased risk of preterm birth and nearly a twelve-fold increased risk of very preterm birth (before 32 weeks). Among twins specifically, only about 36% make it to full term. Roughly half are born in the late preterm window (34 to 36 weeks), and another 14.5% arrive earlier than that. The uterus is simply stretched beyond its capacity sooner, which can trigger contractions or cause the cervix to open early under the added weight and pressure.
Cervical and Uterine Problems
Some women have a cervix that begins to shorten and open well before labor should start, a condition called cervical insufficiency. The cervix normally stays firm and closed until late in pregnancy, but structural weakness at the junction between the cervix and the lower uterus can cause it to give way during the second trimester, often without any contractions at all.
This weakness can be something a woman is born with, such as a uterine shape abnormality, or it can result from previous procedures. Cervical surgeries like cone biopsies or loop excisions, which remove tissue from the cervix, can reduce its structural strength. Tears during a previous delivery can also contribute. In some cases, the cervix simply remodels too early during pregnancy, shortening prematurely and leading to preterm delivery. Doctors can sometimes detect this on ultrasound and intervene with a stitch to hold the cervix closed, but it doesn’t work for every situation.
When Doctors Decide to Deliver Early
Not all preterm births happen spontaneously. In many cases, a medical team decides that delivering the baby early is safer than continuing the pregnancy. These are called medically indicated preterm deliveries, and they account for a significant portion of all premature births.
Preeclampsia, a dangerous rise in blood pressure during pregnancy, is one of the most common reasons. Left untreated, it can progress to seizures, organ damage, or death for the mother. Placental abruption, where the placenta separates from the uterine wall, can cause severe bleeding and cut off the baby’s oxygen supply. Poorly controlled diabetes, severe fetal growth restriction, and certain infections also create situations where the risks of staying pregnant outweigh the risks of an early delivery. In these cases, prematurity is the lesser harm.
Chronic Stress and Racial Disparities
Preterm birth rates are not distributed equally. Poverty influences neighborhood environments, access to nutritious food and quality healthcare, exposure to pollutants, and the burden of chronic health conditions, all of which can affect pregnancy outcomes. But income alone doesn’t explain the gap.
A striking finding from research published in JAMA Network Open showed that Black mothers in the highest income group still experienced higher rates of preterm birth than White mothers in the lowest income group. This disparity persists across every income level, pointing to factors beyond financial resources. Structural racism, discrimination in healthcare settings, chronic psychosocial stress, and unequal access to quality prenatal care all appear to play a role. Chronic stress raises levels of stress hormones that can promote the same inflammatory pathways involved in early labor. The cumulative toll of navigating discrimination over a lifetime, sometimes called “weathering,” may prime the body for earlier delivery regardless of income or education.
Previous Preterm Birth and Recurrence
One of the strongest individual risk factors is having already delivered a baby prematurely. A woman with two prior preterm deliveries has about a 42% chance of delivering early again. If both of those previous births were very early (21 to 31 weeks), the recurrence risk climbs to 57%. Even a single prior preterm birth raises the odds meaningfully compared to a woman whose previous pregnancies all went to term, where the risk sits around 5%.
The pattern of previous deliveries matters too. A woman whose first birth was full-term and second was preterm has about a 21% recurrence risk, while one whose first was preterm but second was full-term drops to about 13%. This suggests that whatever underlying factors contributed the first time, whether cervical weakness, inflammation, or something else, tend to recur but aren’t inevitable.
Other Contributing Factors
Several additional factors increase the likelihood of preterm birth, often by contributing to the same inflammatory or hormonal disruptions described above:
- Maternal age: Women younger than 17 or older than 35 face higher rates of preterm delivery.
- Short interpregnancy interval: Getting pregnant again within six months of a previous delivery raises risk, likely because the body hasn’t fully recovered.
- Smoking and substance use: Tobacco use restricts blood flow to the placenta and promotes inflammation. Cocaine and methamphetamine use also increase risk substantially.
- Underweight or obesity: Both extremes of pre-pregnancy weight are associated with higher preterm birth rates.
- Chronic conditions: High blood pressure, diabetes, kidney disease, and autoimmune disorders all contribute through vascular damage, inflammation, or both.
Globally, preterm birth rates range from 4% to 16% depending on the country, with the highest burden concentrated in southern Asia and sub-Saharan Africa. In many of these regions, limited access to prenatal care, higher rates of infection, and nutritional deficiencies compound the risk. Preterm birth remains one of the leading causes of death in children under five worldwide, making its prevention a major public health priority.