About 1 in 10 babies in the United States is born before 37 weeks of pregnancy, the threshold that defines a preterm birth. Some arrive early because something triggers labor on its own. Others are delivered early by medical teams because staying in the womb has become unsafe for the baby, the mother, or both. The reasons span infections, anatomy, stress, pregnancy complications, and sometimes no identifiable cause at all.
How Normal Labor Gets Triggered Too Soon
In a full-term pregnancy, labor begins when the uterus shifts from a quiet, relaxed state to an active one. Two signals drive this shift: the uterus stretching as the baby grows larger, and hormonal changes in the baby’s own brain signaling that development is complete. Once activated, the uterus becomes sensitive to hormones like oxytocin and prostaglandins that stimulate contractions.
A critical part of this process is the body’s gradual withdrawal from progesterone, the hormone that keeps the uterus calm throughout pregnancy. The uterine muscle doesn’t necessarily see less progesterone in the bloodstream. Instead, it changes how its cells respond to progesterone, effectively “turning down the volume” on the hormone’s calming signal. When this withdrawal happens too early, or when other factors force the uterus into its active phase ahead of schedule, preterm labor begins.
Infection and Inflammation
Infection is one of the most well-understood triggers of early labor. When bacteria reach the uterus, the membranes surrounding the baby, or even the amniotic fluid itself, the body mounts an immune response. That response floods the area with inflammatory molecules that do several things at once: they stimulate prostaglandin production (which causes contractions), they weaken the membranes around the baby (which can cause the water to break early), and they suppress the effect of progesterone on the uterine muscle.
The infections involved aren’t always dramatic. Bacterial vaginosis, a common vaginal imbalance, is one of the most frequently linked conditions. Urinary tract infections, gum disease, and even pneumonia have been associated with preterm birth, because widespread inflammation in the body can reach the uterus through the bloodstream. Some bacteria form biofilms on the fetal membranes that steadily release inflammatory signals, weakening those membranes until they rupture.
Cervical Length and Structural Weakness
The cervix acts as the gateway between the uterus and the outside world, and its ability to stay closed under pressure varies significantly from person to person. A landmark study published in the New England Journal of Medicine measured cervical length by ultrasound at 24 weeks and found a striking dose-response relationship: the shorter the cervix, the higher the risk of delivering early.
Women with a cervix measuring 26 mm or less at 24 weeks (the bottom 10%) had about six times the risk of preterm delivery compared to those with longer measurements. At 22 mm or less (the bottom 5%), the risk jumped to nearly 10 times higher. By 28 weeks, those same short measurements carried even greater risk, with the shortest cervixes predicting up to 25 times the likelihood of early delivery. This isn’t a binary “competent or incompetent” situation. Cervical strength exists on a spectrum, and women at the shorter end face progressively higher odds.
Carrying Twins or More
Over 60% of twins and nearly all triplets or higher-order multiples are born before 37 weeks. The primary driver is simple physics: more than one baby stretches the uterus far beyond what a single pregnancy demands. That overdistension is one of the same signals that triggers labor in full-term pregnancies, but it happens much earlier when the uterus is accommodating two or three growing bodies. The larger placental area in multiple pregnancies also increases the risk of complications like bleeding, which can force an early delivery.
Maternal Health Conditions
High blood pressure disorders, particularly preeclampsia, are among the leading reasons doctors decide to deliver a baby early. When a mother’s blood pressure rises to dangerous levels, it threatens her organs and restricts blood flow to the baby. Delivery is often the only way to resolve the condition.
Gestational diabetes also raises the risk, though the pathway is less straightforward. Blood sugar imbalances can cause acute fetal distress, sometimes requiring emergency delivery. What’s notable is that even women with well-controlled blood sugar levels still face elevated risk. The metabolic disruption of diabetes during pregnancy appears to threaten fetal wellbeing independently of glucose numbers. Women with gestational diabetes who deliver early also tend to have higher rates of excess amniotic fluid and overlapping high blood pressure problems, compounding their risk.
Placental Problems
The placenta is the baby’s lifeline for oxygen and nutrients, and when it fails, early delivery becomes urgent. Placental abruption occurs when the placenta partially or completely separates from the uterine wall before it’s supposed to. This can cut off the baby’s oxygen supply and cause severe bleeding in the mother. It often happens suddenly, and in many cases the only option is immediate delivery regardless of how far along the pregnancy is.
Placenta previa, where the placenta covers the cervix, can also lead to preterm birth. As the cervix naturally begins to thin and prepare for delivery in later pregnancy, a low-lying placenta can tear and bleed, sometimes requiring early cesarean delivery.
Stress and the Cortisol Connection
Chronic stress during pregnancy isn’t just emotionally difficult. It has a measurable biological effect on when labor begins. Stress hormones, particularly cortisol, stimulate the placenta to produce its own stress hormone called CRH. In laboratory studies, human placental cells release CRH in a dose-dependent way in response to cortisol, inflammatory signals, and low oxygen.
Placental CRH plays a central role in timing human labor, and elevated cortisol early in pregnancy is associated with a precocious rise in placental CRH later on. One study found that a larger cortisol spike upon waking in late pregnancy, combined with a failure of that spike to naturally diminish as pregnancy progressed, was significantly linked to shorter pregnancies. This pathway also helps explain documented racial and ethnic disparities in preterm birth rates, since chronic social stress and discrimination produce the same sustained cortisol elevation.
What Can Be Done to Prevent It
For women with a known high-risk profile, specifically those carrying a single baby who’ve had a previous preterm birth and whose cervix measures under 25 mm on ultrasound, two interventions have strong evidence behind them. Vaginal progesterone supplements reduced the risk of delivering before 35 weeks by about 32%, and cut the rate of serious newborn complications by more than half. Cervical cerclage, a stitch placed around the cervix to hold it closed, reduced preterm birth before 35 weeks by about 30% and showed similar benefits at every gestational threshold tested. Research suggests the two approaches are comparably effective for this specific group of patients.
Beyond medical interventions, identifying and treating infections early, managing chronic conditions like diabetes and high blood pressure before and during pregnancy, and reducing exposure to chronic stress all play a role. For multiple pregnancies, closer monitoring allows care teams to catch warning signs before they become emergencies.
Warning Signs of Preterm Labor
Preterm labor doesn’t always announce itself with obvious contractions. The physical signs to watch for include a tightening of the abdomen that comes and goes every 10 minutes or more often, a change in vaginal discharge (especially fluid leaking or any bleeding), pressure in the pelvic area, a low dull backache that doesn’t go away, and cramping that feels like a menstrual period with or without diarrhea. Six or more contractions in a single hour is not normal at any point before full term. These symptoms can be subtle, and many women initially dismiss them as ordinary pregnancy discomfort.