Preterm labor happens when the body begins preparing for delivery before 37 weeks of pregnancy. In the United States, about 1 in 10 babies are born preterm, a rate that has held steady at 10.41% through 2024. The causes range from infections and inflammation to structural differences in the cervix, chronic health conditions, and pregnancy-specific factors like carrying twins. In many cases, multiple triggers overlap, and sometimes no clear cause is found at all.
How the Body Normally Keeps Labor on Hold
For most of pregnancy, the uterus is actively kept quiet. Progesterone, along with several other substances including nitric oxide and relaxin, suppresses the muscle contractions that would otherwise push labor forward. The cervix stays firm and closed, and the membranes surrounding the baby remain intact.
Labor begins when this system of checks breaks down. At the cellular level, calcium floods into uterine muscle cells and triggers a chain reaction that causes the muscle fibers to contract. At the same time, proteins called connexins form bridges between muscle cells, allowing them to contract together in coordinated waves rather than in isolated twitches. This shift from random tightening to organized contractions is what turns a quiet uterus into one that can deliver a baby. When it happens too early, the result is preterm labor.
Infection and Inflammation
Infection is one of the best-understood triggers of preterm labor. Bacteria can travel upward from the vagina into the uterus, reaching the amniotic fluid and membranes. The most commonly identified organism in the amniotic cavity is Ureaplasma urealyticum, a type of genital mycoplasma. Other bacteria found include E. coli, Group B Streptococcus, Gardnerella vaginalis, and Fusobacterium species. Researchers have even detected oral bacteria in amniotic fluid, suggesting that infections originating in the mouth can sometimes reach the uterus through the bloodstream.
When the immune system detects these invaders, it launches an inflammatory response. Immune sensors on cells recognize bacterial components and activate a cascade of signaling molecules. Among the first to appear are inflammatory proteins that stimulate the production of prostaglandins, the same hormones that soften the cervix and trigger contractions during normal labor at term. Other inflammatory molecules recruit white blood cells to the area, which release enzymes that break down the collagen holding the fetal membranes together. This is how infection can cause both premature contractions and early rupture of membranes (when your water breaks too soon).
The inflammation doesn’t have to come from a raging infection. Even a low-grade, “silent” infection with minimal symptoms can produce enough inflammatory signaling to set off labor weeks before the due date. This is one reason preterm labor can seem to come out of nowhere.
A Short or Weakened Cervix
The cervix acts as a physical barrier between the baby and the outside world. During a healthy pregnancy, it stays long and closed until the final weeks. On ultrasound, a normal cervix measures above 25 millimeters. Below that threshold, especially before 28 weeks, the risk of preterm birth rises significantly. Women with a short cervix who are also having contractions have roughly twice the rate of preterm delivery compared to those with a short cervix but no contractions.
Some women have what’s called cervical insufficiency, where the cervix begins to open painlessly and without contractions, sometimes in the second trimester. This can result from previous cervical procedures, injury during a prior delivery, or a structural difference present from birth. Importantly, though, a short cervix isn’t an automatic sentence for early delivery. Even a cervix measuring under 10 millimeters can still hold a pregnancy to term. What seems to matter most is whether the shortened cervix exposes the membranes to bacteria from the vagina, circling back to the role of infection.
Carrying Twins or Multiples
Twin pregnancies carry a dramatically higher risk of preterm birth. Studies report preterm delivery rates between 31% and 63% for twins, with one large analysis finding the rate at just over 60%. The primary reason is uterine overdistension: the uterus stretches beyond what it would for a single baby, and that physical stretch alone can activate the same contraction pathways that infection does. The stretching stimulates the release of prostaglandins and promotes the formation of gap junctions between muscle cells, essentially telling the uterus it’s time to contract even when the calendar says otherwise.
Pregnancies with multiples also carry higher rates of complications like preeclampsia, which independently increases the chance of early delivery.
Hormonal Signals That Speed Up the Clock
The placenta produces a stress hormone called corticotropin-releasing hormone (CRH) throughout pregnancy. In a normal pregnancy, CRH levels rise gradually and peak around the time of delivery. But in women who go on to deliver preterm, CRH levels are measurably higher as early as 16 weeks and climb faster than in women who deliver at term.
CRH drives a feedback loop: it stimulates the baby’s adrenal glands to produce hormones that, in turn, cause the placenta to make even more CRH. This accelerating cycle promotes the production of estrogen relative to progesterone, which is critical because estrogen encourages the uterine muscle to become more excitable and contract-ready. In essence, the hormonal “clock” of pregnancy runs faster in some women, pushing the uterus toward labor-mode weeks or months ahead of schedule.
Maternal Health Conditions
Preeclampsia, a pregnancy complication marked by high blood pressure and organ stress, is a well-established cause of preterm birth. In many cases, doctors must deliver the baby early to protect the mother’s health, making preeclampsia a leading cause of medically indicated preterm delivery rather than spontaneous preterm labor. When preeclampsia occurs alongside gestational diabetes, the risk compounds: women with both conditions have roughly 1.9 times the odds of preterm birth compared to women with neither, even after accounting for age and BMI.
Chronic high blood pressure, kidney disease, and autoimmune conditions like lupus also raise the risk. These conditions share a common thread: they affect blood flow to the placenta or create systemic inflammation, both of which can destabilize the pregnancy.
Smoking, Stress, and Other Lifestyle Factors
Tobacco exposure is strongly linked to preterm birth. Active smoking during pregnancy increases the odds significantly, and even secondhand smoke exposure carries a measurable risk. The mechanism involves reduced blood flow to the placenta, increased inflammation, and direct toxic effects on the membranes and cervix.
Chronic psychological stress and anxiety also contribute, though more modestly. In one large cohort study, higher perceived stress scores and anxiety scores were each associated with a small but consistent increase in preterm birth odds (about 3% higher odds per unit increase on standardized scales). Interestingly, the effects of stress and smoking appear to act independently rather than multiplying each other’s risk.
Other lifestyle-related risk factors include heavy alcohol use, illicit drug use (particularly cocaine and methamphetamine), and being significantly underweight or gaining too little weight during pregnancy.
Prior Preterm Birth and Pregnancy Spacing
A previous preterm delivery is one of the strongest predictors of it happening again. Whatever combination of factors led to early labor the first time, whether cervical, hormonal, or inflammatory, tends to recur.
How soon you become pregnant again also matters. Conceiving within six months of a prior delivery is associated with higher preterm birth risk. Current guidelines recommend waiting at least 18 months between delivery and the next conception, though in the U.S., one in three women become pregnant before that window. The short interval may not allow the uterus and cervix to fully recover, or it may deplete nutritional reserves like folate and iron that support a healthy pregnancy.
Racial Disparities in Preterm Birth
Preterm birth does not affect all populations equally. In 2022, the preterm birth rate among Black women was 14.6%, roughly 50% higher than the rate among White women (9.4%) or Hispanic women (10.1%). This gap persists even after controlling for income, education, and access to prenatal care, pointing to deeper factors: chronic stress from systemic racism, differences in the inflammatory and hormonal environment shaped by lifelong exposure to discrimination, neighborhood-level environmental hazards, and disparities in the quality of healthcare received.
Recognizing the Warning Signs
Preterm labor doesn’t always announce itself with obvious pain. The contractions may feel like a painless tightening or hardening of the abdomen that comes at regular intervals. Other signs include a change in vaginal discharge (becoming watery, bloody, or mucus-like), an increase in the amount of discharge, constant low back pain, pelvic pressure that feels like the baby is pushing down, and abdominal cramps that may feel like period pain.
The key distinction is pattern and persistence. Occasional tightening (Braxton Hicks contractions) is normal in the second and third trimesters. Regular or frequent tightening that doesn’t stop when you change position or rest is not. If you notice any of these signs before 37 weeks, contact your provider or go to the hospital without waiting to see if they resolve on their own. Early intervention can sometimes delay delivery long enough to make a significant difference for the baby.