Premature birth happens when a baby arrives before 37 weeks of pregnancy, and it affects about 1 in every 10 infants born in the United States. The causes range from infections and placental problems to chronic health conditions in the mother, and in many cases, no single cause can be pinpointed. About half of all preterm births begin with spontaneous early labor, roughly a quarter follow an early rupture of the amniotic sac, and the remaining quarter are planned early deliveries made necessary by serious complications.
How Preterm Birth Is Classified
Not all premature births carry the same level of risk. The World Health Organization breaks them into three categories based on how early the baby arrives. Moderate to late preterm births happen between 32 and 37 weeks and account for the largest share. Very preterm births occur between 28 and 32 weeks. Extremely preterm births, the most medically serious, happen before 28 weeks. The earlier a baby is born, the greater the chance of complications with breathing, feeding, and organ development.
Infections and Inflammation
Infection is one of the best-understood triggers for early labor. Certain bacteria, particularly Gardnerella vaginalis and Ureaplasma species, can travel upward from the vagina into the uterus during pregnancy. Once there, they provoke an inflammatory response that can weaken the membranes surrounding the baby or trigger contractions well before the due date. This “ascending infection” pathway is well documented in research and helps explain why urinary tract infections and bacterial vaginosis are consistently linked to higher preterm birth rates.
What makes this tricky is that not all infection-related preterm births involve bacteria reaching the uterus. Some appear to be driven by inflammation alone. The presence of certain bacterial communities in the vagina during pregnancy is associated with increased risk even without a full-blown intrauterine infection. This means the body’s immune reaction to imbalanced vaginal bacteria can be enough to set early labor in motion.
Preeclampsia and High Blood Pressure
Preeclampsia is a pregnancy complication marked by dangerously high blood pressure, and it is one of the most common reasons doctors decide to deliver a baby early. The condition appears to start with the placenta. When the placenta doesn’t form a strong enough connection to the blood vessels of the uterus, it doesn’t receive adequate blood flow. In response, the placenta releases chemicals that damage the lining of blood vessels throughout the mother’s body, causing them to constrict. That constriction drives blood pressure up and can damage the kidneys, liver, and brain.
Left untreated, preeclampsia can progress to seizures (eclampsia) or organ failure. In severe cases, the only effective treatment is delivering the baby, even if it means a premature birth. This makes preeclampsia a leading cause of medically indicated preterm delivery, especially in the third trimester.
Placental Problems
The placenta is the baby’s lifeline, and two complications involving it frequently lead to early delivery.
Placenta previa occurs when the placenta covers part or all of the cervix. As the uterus stretches and the lower segment thins during the third trimester, the placenta can bleed heavily. Most cases require a cesarean delivery, and if bleeding becomes severe before term, the baby must be delivered early.
Placental abruption is more sudden. The placenta partially or fully separates from the uterine wall, cutting off the baby’s oxygen and nutrient supply. The hallmark symptom is dark red vaginal bleeding with abdominal pain, typically in the third trimester or during labor. If the abruption is large enough to affect the baby, an emergency cesarean is usually necessary regardless of gestational age.
Carrying Twins or Multiples
Multiple pregnancies dramatically increase the odds of preterm birth. In the United States, 62.3% of babies from multiple pregnancies (twins, triplets, or more) are born preterm, compared to 8.8% of singletons. The uterus simply runs out of room sooner, and the greater demands on the placenta raise the risk of complications like preeclampsia and growth restriction. The more babies being carried, the earlier delivery tends to happen.
Cervical Insufficiency
Your cervix acts as a closed door between the uterus and the vagina, and it’s meant to stay shut until labor begins near your due date. In cervical insufficiency, the cervix softens, shortens, or opens too early, sometimes in the second trimester, without any contractions or pain. Because there are often no warning symptoms, the condition can lead to very early delivery before the baby’s organs are fully developed. A history of cervical surgery, prior second-trimester losses, or certain congenital differences in the cervix can increase the risk. Once identified, a cervical stitch (cerclage) or progesterone treatment can help keep the cervix closed longer in future pregnancies.
Smoking and Substance Use
Smoking during pregnancy raises preterm birth risk in a clear dose-dependent pattern: the more cigarettes per day, the higher the risk. A large analysis of 25 million mother-infant pairs found that women who smoked 1 to 5 cigarettes per day during the first trimester had about a 31% higher chance of preterm delivery compared to nonsmokers. For women smoking 20 or more cigarettes daily, that risk jumped to 53% higher. The numbers were even steeper for smoking during the second trimester, with heavy smokers facing a 59% increase.
Smoking constricts blood vessels supplying the placenta, reducing oxygen and nutrients to the baby. It also increases inflammation and raises the likelihood of placental abruption. Other substances, including alcohol and certain recreational drugs, carry their own risks for preterm delivery through similar vascular and inflammatory effects.
Pregnancy Spacing
Getting pregnant again too soon after a previous delivery is an often-overlooked risk factor. The recommended minimum gap between delivering one baby and conceiving the next is 18 months. Shorter intervals don’t give the body enough time to replenish nutritional stores, heal uterine tissue, and resolve the inflammation from the prior pregnancy. This is especially relevant for women who have had a cesarean delivery, where the uterine scar needs time to fully strengthen.
Other Contributing Factors
Several additional factors raise the likelihood of preterm birth, and many of them overlap or compound one another. Chronic conditions like diabetes and kidney disease affect blood vessel health and placental function. Being under 17 or over 35 increases risk at both ends of the age spectrum. Black women in the United States face significantly higher preterm birth rates than white women, a disparity driven by systemic health inequities, chronic stress, and differences in access to care rather than biology alone.
A prior preterm birth is one of the strongest individual predictors. Women who have delivered early once are substantially more likely to do so again, which is why obstetricians monitor subsequent pregnancies more closely with cervical length measurements and, in some cases, progesterone supplementation. Stress, both acute and chronic, also plays a role by elevating hormones that can trigger contractions and weaken immune defenses against infection.
In many preterm births, especially spontaneous ones, multiple risk factors interact simultaneously. A woman might have a mild vaginal infection, a slightly short cervix, and elevated stress levels, none of which alone would cause early labor, but together they tip the balance. This is part of why preterm birth remains difficult to predict and prevent despite decades of research. The U.S. preterm birth rate declined slightly from 2021 to 2022, falling 1% to 10.4%, but it remains stubbornly high compared to other high-income countries.