Premature birth, defined as delivery before 37 weeks of gestation, affects about 1 in every 10 infants born in the United States. The 2024 preterm birth rate was 10.41%, a figure that has remained essentially stable since 2021. There is no single cause. Premature birth results from several overlapping biological pathways, including infection, placental problems, cervical weakness, hormonal shifts, stress, and lifestyle factors.
How the Body Normally Keeps Labor at Bay
For most of pregnancy, the hormone progesterone keeps the uterus quiet. It does this by blocking the signals that would otherwise trigger contractions, suppressing inflammatory chemicals, and promoting muscle relaxation in the uterine wall. When it’s time for a full-term birth, the body undergoes a two-phase shift: first the uterus “activates” in response to stretching from the growing baby and hormonal signals from the fetus, then it’s “stimulated” by a rising cascade of contraction-promoting hormones and chemicals like oxytocin and prostaglandins.
Premature birth happens when something triggers this cascade too early. The four most common pathways are infection or inflammation, problems with blood flow to the placenta, physical overdistension of the uterus (as with twins or excess amniotic fluid), and high maternal stress.
Infection and Inflammation
Infections in the uterus or reproductive tract are one of the most well-established triggers for preterm labor. When bacteria reach the uterine lining or the membranes surrounding the baby, the immune system responds with inflammation. That inflammation produces the same prostaglandins that drive normal labor contractions, essentially jumpstarting labor weeks or months too soon.
The most commonly identified bacteria in intrauterine infections are Ureaplasma urealyticum, Fusobacterium species, and Mycoplasma hominis. Urinary tract infections also play a role: treating UTIs that have no symptoms (asymptomatic bacteriuria) during pregnancy significantly reduces preterm birth risk. Interestingly, the same benefit has not been clearly demonstrated for bacterial vaginosis, despite its frequent association with preterm delivery.
Placental Abruption and Blood Flow Problems
The placenta is the baby’s sole source of oxygen and nutrients. When it partially separates from the uterine wall before delivery, a condition called placental abruption, the consequences can be severe enough to require immediate early delivery.
Abruption happens when blood vessels connecting the placenta to the uterus tear away. The uterus is a muscle and stretches easily, but the placenta is made of less elastic tissue. When the uterus stretches suddenly, the connection can rupture. Blood pools between the placenta and the uterine wall, and that accumulating blood drives further separation. High blood pressure, substance use, and conditions that cause rapid uterine stretching all increase the risk. If the separation is moderate to severe, delivery becomes necessary regardless of how far along the pregnancy is.
Placenta previa, where the placenta covers or sits near the cervical opening, can also lead to premature delivery when it causes significant bleeding.
Cervical Insufficiency
In some pregnancies, the cervix begins to open painlessly and without contractions, sometimes in the second trimester. This is called cervical insufficiency, and it can lead to premature rupture of membranes, mid-trimester pregnancy loss, or early delivery.
The condition is typically diagnosed when the cervix measures shorter than 25 mm on ultrasound before 24 weeks, or when a physical exam reveals painless dilation without labor. Some cases stem from congenital differences in the cervix, including uterine structural anomalies or connective tissue conditions like Ehlers-Danlos syndrome. Others are acquired from prior cervical procedures or trauma. A history of second-trimester losses or prior preterm births is a significant risk factor.
For women with a short cervix and a history of spontaneous preterm birth, two interventions have strong evidence behind them: vaginal progesterone supplementation and cervical cerclage, a stitch placed around the cervix to hold it closed. A meta-analysis of randomized trials found that both approaches are equally effective at reducing preterm birth before 35 weeks. Cerclage reduced preterm birth rates at multiple thresholds (37, 35, 32, and 28 weeks), while vaginal progesterone reduced risk before 35 and 32 weeks and also lowered neonatal intensive care admissions.
Stress and the Cortisol Feedback Loop
Maternal stress is not just a vague risk factor. It operates through a specific hormonal mechanism. When you’re stressed, your body produces cortisol. In most tissues, cortisol inhibits the release of corticotropin-releasing hormone (CRH), a chemical that helps regulate the stress response. But in the placenta, the opposite happens: cortisol stimulates CRH production. This creates a positive feedback loop where stress hormones and placental CRH amplify each other throughout pregnancy.
Research has shown that elevated cortisol early in pregnancy is associated with a premature surge in placental CRH later in pregnancy, and this early rise in CRH is linked to preterm delivery. Psychosocial stress, including financial hardship, relationship conflict, and discrimination, correlates with higher levels of the pituitary-adrenal hormones that feed this cycle. The biological stress response is also stronger earlier in pregnancy than later, which may explain why major stressors in the first and second trimesters carry particular risk.
Pregnancy Spacing
Conceiving again too soon after a previous birth is an independent risk factor for preterm delivery. Women with an interpregnancy interval of six months or less have roughly 40% higher odds of preterm birth and 84% higher odds of premature membrane rupture compared to those who wait at least 24 months. The World Health Organization recommends waiting at least 24 months after a live birth before conceiving again, and U.S. research has found that an interval of 18 to 23 months carries the lowest risk of preterm birth, low birth weight, and small-for-gestational-age babies.
The reason likely involves maternal recovery. Pregnancy depletes nutrient stores, particularly folate and iron, and the uterus and cervix need time to fully heal. A short interval may not allow the body to replenish those reserves or complete tissue repair.
Multiple Pregnancies and Uterine Overdistension
Carrying twins, triplets, or higher-order multiples significantly increases the risk of premature birth simply because the uterus stretches more and faster than it does with a single baby. That stretching is one of the normal signals that activates labor. When it happens earlier due to more than one baby or excess amniotic fluid (polyhydramnios), it can trigger contractions well before term. More than half of twins are born before 37 weeks.
Other Contributing Factors
Several additional factors raise preterm birth risk, often by feeding into the pathways described above. Smoking constricts blood vessels supplying the placenta, increasing the risk of abruption and growth restriction. Alcohol and drug use, particularly cocaine, are linked to both placental problems and preterm labor. Chronic conditions like diabetes and high blood pressure can compromise placental blood flow. Being underweight or having a very high body mass index before pregnancy also increases risk.
Age plays a role at both ends: teenagers and women over 35 face higher rates of preterm delivery. And some risk factors are not modifiable. Black women in the United States experience preterm birth at significantly higher rates than white women, a disparity that persists even after controlling for income, education, and health behaviors, pointing to the cumulative biological effects of chronic stress and systemic inequity.
Warning Signs of Preterm Labor
Preterm labor can develop suddenly, and recognizing the signs early makes a meaningful difference. The key symptoms to watch for are regular or frequent tightening in the abdomen (contractions that come at predictable intervals, not the irregular Braxton Hicks type), a gush or steady trickle of fluid from the vagina suggesting the membranes have ruptured, vaginal discharge that is watery, bloody, or mucus-filled, and vaginal spotting or light bleeding. Persistent low back pain and pelvic pressure that doesn’t let up can also signal early labor. If any of these develop before 37 weeks, getting evaluated quickly gives providers the best window to intervene.