Preterm labor is when your body starts preparing for delivery before 37 weeks of pregnancy. It affects roughly 1 in 10 pregnancies in the United States, with the preterm birth rate holding at 10.41% as of 2024. Not all preterm labor leads to preterm birth, and the earlier it’s caught, the more options doctors have to delay delivery and protect the baby.
How It Differs From Normal Labor
Full-term pregnancy runs 39 to 40 weeks. Labor that begins between 20 and 37 weeks is classified as preterm. The distinction matters because every additional week a baby stays in the womb significantly improves health outcomes, particularly for lung and brain development. Preterm labor can progress to delivery within hours or can be slowed or stopped with medical intervention, depending on how far along it is and what’s causing it.
Preterm labor can happen spontaneously or result from a medical decision to deliver early due to complications like severe high blood pressure or problems with the placenta. Spontaneous preterm labor is driven by a few core mechanisms: inflammation in the uterine lining, bleeding behind the placenta, abnormal stretching of the uterus (common with twins or triplets), and conditions that stress the mother or baby.
Symptoms to Recognize
The tricky part of preterm labor is that some of its symptoms overlap with normal pregnancy discomfort. The key difference is a pattern of regular, repeated tightening in your belly. Specifically, watch for:
- Regular contractions: frequent tightening sensations in your abdomen that come at predictable intervals
- Low back pain: a constant, dull ache that doesn’t go away with position changes
- Pelvic pressure: a feeling of heaviness or downward pressure in your lower belly
- Vaginal changes: spotting, light bleeding, watery fluid, or mucus-like discharge
- Fluid leaking: a gush or slow trickle, which may mean the amniotic sac has broken
Any combination of these before 37 weeks warrants immediate contact with your healthcare provider. You don’t need to wait for contractions to become painful or for multiple symptoms to appear at once. A single symptom like fluid leaking is enough to call right away.
Who Is Most at Risk
The single strongest predictor of preterm labor is having had a preterm birth before. Women with one prior preterm birth are two to three times more likely to deliver early again, and the risk climbs with each occurrence.
Beyond personal history, several other factors raise the likelihood:
- Cervical factors: a short cervix (measured by ultrasound), early cervical dilation, or previous cervical procedures
- Pregnancy-specific issues: carrying twins or more, vaginal bleeding, or infections during pregnancy
- Age: being younger than 17 or older than 35
- Lifestyle and health: smoking during pregnancy, low pre-pregnancy weight, poor nutrition, and inadequate prenatal care
- Chronic conditions: high blood pressure disorders, diabetes, and autoimmune diseases can reduce blood flow to the placenta, increasing risk
Black women face a disproportionately higher risk of preterm birth, a disparity that persists even after controlling for income, education, and access to care. The reasons are complex and likely involve chronic stress, systemic inequities in healthcare, and biological factors that researchers are still working to untangle.
How Preterm Labor Is Diagnosed
When you arrive at a hospital with possible preterm labor, the evaluation typically involves two things: checking your cervix and running a test on vaginal fluid.
Cervical length is measured with a transvaginal ultrasound. A cervix shorter than 25 millimeters is generally considered “short” and signals higher risk, though the exact cutoff varies by situation. For universal screening of women without symptoms, 20 millimeters or less is the threshold that prompts action. Every millimeter of shortening incrementally raises the odds of early delivery.
The second tool is a protein test on vaginal secretions. This protein is normally present in early pregnancy but shouldn’t appear in significant amounts during the middle weeks. A level above a certain threshold suggests the body is preparing for delivery. The test is most useful as a negative result: if the protein isn’t elevated, the chance of delivering within the next two weeks is very low. That negative result can spare you unnecessary hospitalization and medications.
What Happens During Treatment
The goals of treatment are straightforward: delay delivery long enough to give the baby’s organs more time to mature, and administer medications that speed up that maturation from the outside.
Corticosteroids are the most important intervention for the baby. Given as injections to the mother, they cross the placenta and accelerate lung development. They’re recommended for women at risk of delivering within seven days, from as early as 23 weeks through 36 weeks and 6 days. A single course is standard. These steroids dramatically reduce the risk of breathing problems, bleeding in the brain, and other serious complications in premature newborns.
To buy time for those steroids to work (they need about 48 hours for full effect), doctors may use medications called tocolytics that temporarily slow or stop contractions. These drugs are not intended to prevent preterm birth altogether. They’re a short-term bridge. Tocolytics are not used in the late preterm period (after 34 weeks) and are never used to delay a delivery that needs to happen for medical reasons, like severe preeclampsia.
Why Gestational Age Matters So Much
The outcomes for a baby born early depend enormously on how early. Even a difference of one or two weeks can change the picture dramatically.
At 24 weeks, the situation is dire. In one large study, nearly a third of babies born at this age did not survive, and among those who did, every single one experienced significant health complications. Major problems at this stage include bleeding in the brain, chronic lung disease, and serious intestinal damage.
By 28 weeks, the survival rate jumps to 98%. Major complications drop to about 22%, though roughly three-quarters of babies still need treatment for issues like breathing support or jaundice. Only about 3% go home without any complications at all.
At 32 weeks, the picture improves further. Death is rare (0.2%), and major complications fall below 9%. About 15% of babies born at this stage avoid all tracked complications entirely. Most issues at this point are manageable, things like temporary breathing difficulty or the need for light therapy to treat jaundice.
After 34 weeks, babies generally do well, though they still face higher rates of feeding difficulties, temperature regulation problems, and short NICU stays compared to full-term infants. The morbidity rates continue dropping steadily through weeks 35 and 36.
Prevention for High-Risk Pregnancies
If you’ve had a prior preterm birth, your provider will likely monitor your cervical length with regular ultrasounds starting around 16 weeks. What happens next depends on what those measurements show.
Vaginal progesterone, a hormone supplement inserted nightly, may be offered if you have both a history of preterm birth and a shortened cervix. It helps maintain the cervix and reduce the chance of early delivery in this specific group. Progesterone is not recommended if your cervix length is normal, even with a prior preterm birth. The older approach of weekly progesterone injections is no longer recommended for preventing recurrent preterm birth.
For women whose cervix is very short (below 15 millimeters), a cervical cerclage may be an option. This is a stitch placed around the cervix to hold it closed, and it has been shown to significantly reduce the rate of delivery before 35 weeks in the right candidates. The decision involves weighing your cervical measurements, your pregnancy history, and what treatments you’ve had before. It’s a conversation between you and your provider, not a one-size-fits-all protocol.
For all pregnancies, the basics of prevention are less dramatic but still meaningful: consistent prenatal care, not smoking, maintaining adequate nutrition, and managing chronic conditions like diabetes or high blood pressure before and during pregnancy.