Preterm birth

Lore & Background

Preterm birth is categorized by gestational age: extreme preterm (less than 28 weeks), very early preterm (28–32 weeks), early preterm (32–34 weeks), and late preterm (34–36 weeks). Symptoms of preterm labor include uterine contractions more often than every ten minutes, leaking of fluid from the vagina before 37 weeks, vaginal bleeding in the third trimester, heavy pelvic pressure, or abdominal or back pain. In some cases, the cervix dilates prematurely without pain or perceived contractions. The cause of spontaneous preterm birth is often unknown, but risk factors include diabetes, high blood pressure, multiple gestation, obesity or underweight, vaginal infections, air pollution exposure, tobacco smoking, and psychological stress.

Reader's Guide

Preterm birth is a critical global health issue. Survival rates increase with gestational age: at 22 weeks about 6%, at 23 weeks 26%, at 24 weeks 55%, and at 25 weeks about 72%. Prevention in at-risk women may involve taking progesterone during pregnancy; evidence does not support bed rest. Corticosteroid treatment between 24 and 37 weeks may improve outcomes, and medications like nifedipine can delay delivery to allow transfer to better care. Postnatal care includes keeping the baby warm, supporting feeding, treating infections, and assisting breathing, sometimes with intubation. Rates of preterm birth vary by country and have increased in many nations between the 1990s and 2010s.

Did You Know?

• Late preterm birth (34–36 weeks) accounts for 75% of all preterm births.
• The chance of survival at 22 weeks is about 6%, while at 25 weeks it is about 72%.
• In the U.S., Black women have a preterm birth rate of about 14%, roughly 1.5 times higher than the white rate of about 9.1%; in the UK, the disparity varies across data sources.
• Approximately 0.5% of births are extremely early periviable births (20–27 weeks), but most preterm deaths occur at later gestations.

The Spectrum of Prematurity

Preterm birth is defined as delivering a baby before completing 37 weeks of gestation, compared to the roughly 40 weeks that characterize a full-term pregnancy. Medical professionals further divide this window into distinct categories that carry progressively different levels of vulnerability. An extreme preterm birth occurs before 28 weeks, very early preterm spans 28 to 32 weeks, early preterm covers 32 to 34 weeks, and late preterm falls between 34 and 36 weeks. In everyday language, these infants are called preemies in American English or premmies in Australian English. The sheer volume of preterm births is staggering: roughly 15 million babies each year, representing between five and eighteen percent of all deliveries worldwide. Late preterm infants make up the overwhelming majority, accounting for three-quarters of all preterm births. National rates vary considerably; the United Kingdom reports about 7.9 percent of babies arriving early, whereas the United States sees 12.3 percent. At the most extreme end, periviable births between 20 and 25 weeks represent only about half a percent of all deliveries, yet they account for the largest share of preterm-related deaths.

The Elusive Causes and Risk Landscape

One of the most frustrating aspects of preterm birth is that in many spontaneous cases, no single cause can be identified. Researchers have proposed four principal biological pathways that may trigger early labor: premature activation of the fetal endocrine system, overdistension of the uterus such as from placental abruption, bleeding in the decidua, and inflammation or infection within the uterus. In practice, multiple factors often converge simultaneously, making it difficult to isolate a single trigger, and research on the cervix in particular remains limited. Maternal risk factors span a wide range. Diabetes, elevated blood pressure, carrying more than one baby, being significantly overweight or underweight, vaginal infections, exposure to air pollution, tobacco use, and chronic psychological stress all elevate the likelihood of early delivery. Additional maternal considerations include being very young or older in age, extremes of body mass index, short intervals between pregnancies, endometriosis, prior miscarriages or surgical abortions, unintended conception, undiagnosed celiac disease, fertility challenges, heat exposure, and genetic predispositions. Occupational stress, physically demanding work, and long hours have also been linked, though study results in this area remain inconsistent.

Medical Interventions and the Race Against Time

When preterm labor is detected, a narrow window of medical intervention can make a meaningful difference. For women at elevated risk, taking the hormone progesterone during pregnancy has shown potential to prevent early birth, whereas bed rest—once widely prescribed—lacks supporting evidence. If delivery appears likely between 24 and 37 weeks, a course of corticosteroids can improve the baby's outcomes. Tocolytic medications such as nifedipine may buy precious hours to delay delivery, giving the mother time to reach a facility with a neonatal intensive care unit and allowing the corticosteroids to take full effect. Medical induction or cesarean section before 39 weeks is generally discouraged unless a condition like preeclampsia demands it. Once the baby arrives, immediate care focuses on maintaining body temperature through skin-to-skin contact or an incubator, establishing feeding via breast milk or formula, managing any infections, and supporting respiration. Some infants require intubation to breathe. Recognizing true preterm labor—four or more contractions within an hour accompanied by cervical dilation and effacement, as opposed to false labor—remains critical, as premature rupture of membranes can precipitate dangerous chorioamnionitis for both mother and child.

A Global Crisis and the Narrowing Survival Gap

Preterm birth stands as the leading cause of infant mortality worldwide. Yet in many countries, the incidence of premature delivery actually rose between the 1990s and the 2010s. Survival odds are intimately tied to gestational age. At 22 weeks, the chance of surviving is roughly six percent; by 23 weeks it climbs to 26 percent, at 24 weeks to 55 percent, and at 25 weeks to about 72 percent. The probability of surviving without any long-term complications is even lower. Premature infants face elevated risks of cerebral palsy, developmental delays, hearing impairment, and visual problems, with the severity scaling inversely with the number of weeks in the womb. Identifying high-risk mothers early and connecting them to specialized neonatal care remains one of the most promising strategies for reducing this global burden.