Premature birth causes: What to know

Hearing the words “preterm labor” or “premature birth” can make any parent-to-be feel like time has suddenly sped up in the worst possible way.
A full-term pregnancy is supposed to last about 40 weeks, but in the United States, roughly 1 in 10 babies is born too early, before 37 completed weeks of pregnancy.
Premature birth is a leading cause of newborn illness and death worldwide, and it can affect families from every background.

The tricky part? There isn’t just one simple cause. Premature birth is usually the result of a mix of medical, lifestyle, and social factors, and sometimes it happens with no clear explanation at all.
Understanding the most common causes and risk factors can’t guarantee a full-term pregnancy, but it can help you recognize what’s going on in your body, ask better questions at appointments, and get support sooner.

Quick note: This article is for education and general awareness. It isn’t a substitute for medical advice, diagnosis, or treatment. Always talk with your healthcare provider about your specific situation.

What counts as premature birth?

A baby is considered premature (or preterm) when they’re born before 37 weeks of pregnancy. Researchers and organizations like the World Health Organization further break that down into categories:​

• Late preterm: 34 weeks 0 days to 36 weeks 6 days
• Moderate preterm: 32 weeks 0 days to 33 weeks 6 days
• Very preterm: 28 weeks 0 days to 31 weeks 6 days
• Extremely preterm: Less than 28 weeks

The earlier a baby is born, the higher the risk of complications. But even late-preterm babies (just a few weeks early) can have challenges with breathing, feeding, and staying warm, and may need extra monitoring.

The big picture: why premature birth happens

From a medical perspective, premature birth falls into two broad categories:

• Spontaneous preterm birth: Labor starts on its own, or the water breaks early (preterm premature rupture of membranes, or PPROM).
• Medically indicated preterm birth: Doctors recommend delivering early by induction or cesarean section because continuing the pregnancy poses serious risks to the mother, the baby, or both (for example, severe preeclampsia or major placental problems).

In many cases, there are identifiable risk factors, like high blood pressure, infection, or carrying twins. But frustratingly, a significant number of preterm births happen in people who had no obvious risk factors at all.
That’s one reason preterm birth is such a focus of global health research: it’s common, serious, and still not fully understood.

Medical causes and risk factors for premature birth

1. Problems with the cervix, uterus, or placenta

Some structural issues with the reproductive system can make it harder to stay pregnant until term:

• Short cervix or cervical insufficiency: A cervix that’s shorter than usual or opens too early can’t always keep the pregnancy supported, leading to preterm labor or early rupture of membranes. Providers often monitor cervical length via ultrasound in people at higher risk.
• Uterine anomalies: A uterus that’s unusually shaped (such as bicornuate or unicornuate) or scarred from previous surgery can increase the risk of preterm birth.
• Placental problems: Conditions like placenta previa (when the placenta covers the cervix) or placental abruption (when the placenta separates from the uterus too early) may make early delivery the safest choice.

These conditions don’t automatically mean you will have a preterm birth, but they do put you in a higher-risk category that calls for closer monitoring and, in some cases, preventive treatment.

2. Infections and inflammation

Infection is one of the most importantand sometimes invisiblecauses of preterm birth. The problem isn’t just the infection itself, but the inflammation it triggers inside the uterus and surrounding tissues.
Some studies suggest that infection and inflammation may contribute to up to about 40% of spontaneous preterm births.

Infections linked to preterm birth can include:

• Genital tract infections (such as bacterial vaginosis, chlamydia, or gonorrhea)
• Infections of the amniotic fluid and membranes around the baby
• Untreated urinary tract infections
• Systemic infections that make the parent seriously ill

Sometimes these infections cause obvious symptomslike burning during urination or unusual dischargebut other times, they’re completely silent.
That’s why routine prenatal screening and speaking up about new symptoms (even if they feel minor) is such a big deal.

3. Chronic health conditions

Conditions that affect overall health can also impact how the body handles pregnancy. Common examples include:

• High blood pressure and preeclampsia: These can damage blood vessels and reduce blood flow to the placenta, sometimes making early delivery medically necessary.
• Diabetes (pre-existing or gestational): Poorly controlled blood sugar is linked with complications that may lead to preterm birth.
• Kidney or heart disease: These can make it harder for the body to keep up with the extra demands of pregnancy.
• Autoimmune or clotting disorders: These may affect the placenta, increase the risk of preeclampsia, or lead to growth problems in the baby that make early delivery safer.

The key message: having a chronic condition doesn’t mean you’re destined to deliver early, but it does mean you and your healthcare team need a planideally before pregnancy or as early as possible.

4. Multiple pregnancy (twins, triplets, and beyond)

Carrying more than one baby is one of the strongest known risk factors for preterm birth. Twins, triplets, and higher-order multiples stretch the uterus more quickly and place extra demands on the placenta and the parent’s body.

Studies consistently show that a large share of twins and most triplets are born before 37 weeks.
If you’re pregnant with multiples, your provider will likely schedule more frequent visits, ultrasounds, and possibly consultations with a maternal–fetal medicine specialist.

5. Previous premature birth

One of the strongest predictors of a future preterm birth is having had one before.
That doesn’t mean it will happen again, but it does put you in a higher-risk group.

For people with a history of spontaneous preterm birth, providers may:

• Monitor the cervix regularly by ultrasound
• Recommend vaginal progesterone if the cervix is short or other risk factors are present
• Consider a cervical cerclage (a stitch that helps keep the cervix closed) in selected cases

The science on preventing recurrent preterm birth is evolving. For example, the injectable progesterone 17-OHPC that was once commonly used has had its approval withdrawn by the U.S. FDA after newer evidence questioned its benefit, and current guidance focuses more on vaginal progesterone in specific situations.

Lifestyle, environmental, and social factors

Health is never just about organs and lab tests. The conditions in which people live, work, and carry pregnancies matter a lot. Several non-medical factors are associated with a higher risk of premature birth:​

• Smoking and vaping: Tobacco exposure affects blood flow to the placenta and is strongly linked to preterm birth and low birth weight.
• Alcohol and illicit drugs: These can harm the developing baby and increase the risk of early labor and other complications.
• Extreme body weight: Being significantly underweight or severely overweight before pregnancy is associated with a higher risk of premature birth.
• Heavy physical labor or long hours on your feet: Demanding work conditions may contribute to preterm labor in some cases.
• Chronic stress, trauma, or intimate partner violence: These aren’t just “in your head”they can affect hormones, blood pressure, and inflammation in ways that may influence pregnancy outcomes.
• Lack of prenatal care or difficulty accessing healthcare: Without regular checkups, infections and complications are more likely to go unnoticed until they’re serious.
• Environmental exposures: Air pollution and certain toxins are being studied as potential contributors to preterm birth risk.

Many of these factors are tied to social and economic inequalities, not personal failure. That’s why public health strategies aimed at reducing preterm birth also focus on things like improving access to healthcare, reducing poverty, and supporting mental health.

What does not usually cause premature birth?

While there’s a lot that can increase risk, there are also common myths that deserve a gentle debunking. In an otherwise uncomplicated pregnancy, the following are usually considered safe (but always check with your provider):

• Having sex (unless you’ve been told otherwise)
• Moderate exercise approved by your healthcare provider
• Riding in a car or airplane
• Working at a typical desk job

These everyday activities don’t cause premature birth on their own. But if you have specific risk factorslike a short cervix, vaginal bleeding, or a history of preterm laboryour care team may recommend adjustments.

Can premature birth be prevented?

Unfortunately, there’s no guaranteed way to prevent preterm birth. However, many strategies can lower risk or catch problems earlier, especially if you know you’re in a higher-risk group.

Before pregnancy

• Get chronic conditions under control: Work with your healthcare provider to manage blood pressure, diabetes, and other health issues.
• Quit smoking and limit alcohol: Stopping before pregnancy (or as early as possible) gives your body time to recover.
• Reach a healthier weight: Even modest changes can help.
• Review medications: Make sure your prescriptions and supplements are pregnancy-safe.

During pregnancy

• Go to all prenatal visits: These checkups help catch high blood pressure, gestational diabetes, infections, and growth problems earlier.
• Report new or unusual symptoms: Pelvic pressure, cramping, fluid leaks, or bleeding should be checked right away.
• Screen and treat infections: Follow through on recommended tests and treatments.
• Ask about targeted treatments if you’re high risk: Depending on your situation, your provider may suggest vaginal progesterone, a cervical cerclage, low-dose aspirin (for preeclampsia risk), or extra monitoring.
• Protect your mental health: Therapy, support groups, and stress-reduction strategies aren’t “optional extras”they’re part of whole-person care.

None of these steps offer a 100% guarantee, but together they can stack the odds more in your favor and give your care team more information to act on.

Warning signs: when to call your provider or go in urgently

If you’re pregnant, especially in the second half of pregnancy, contact your provider or go to the hospital if you notice:

• Regular tightening or contractions that don’t go away with rest or hydration
• Menstrual-like cramping, pelvic pressure, or low back pain that keeps coming and going
• Leaking fluid from the vagina (especially a gush of clear fluid)
• Vaginal bleeding or more discharge than usual, especially if it’s watery, mucus-like, or tinged with blood
• Fewer baby movements than usual
• Severe headache, vision changes, or sudden swelling in the face or hands (possible preeclampsia signs)

You never have to worry about “bothering” your provider. It’s their job to assess what’s going on, and it’s always better to be checked and reassured than to wait and see when something serious might be brewing.

Lived experiences: what causes of premature birth can look like in real life

Statistics and risk-factor lists are helpful, but they don’t capture the emotional rollercoaster that often comes with premature birth.
While every story is different, a few common themes show how the medical causes above play out in real life.

A surprise infection that changed the timeline

Imagine feeling pretty good in your second trimestertired, sure, but overall okay. Then you start having mild cramps and a bit more discharge than usual.
It’s easy to assume it’s just “one of those pregnancy things.” But at a checkup, your provider finds signs of a genital tract infection and possible early membrane changes.
Even with prompt treatment, inflammation can sometimes trigger contractions or weakening of the membranes around the baby.

Parents who’ve been through this often describe feeling blindsided. They didn’t “do anything wrong”they just happened to have an infection that their body responded to strongly.
Afterward, many become passionate about listening to their bodies and encouraging others to speak up about even small changes.

Living with chronic conditions and constant monitoring

For someone with long-standing high blood pressure or type 1 diabetes, pregnancy can feel like a high-stakes balancing act.
There are more appointments, more lab tests, more ultrasounds, and lots of conversations about what’s safest for both parent and baby.

Some people need to adjust medications, change diets, or check blood sugar several times a day. Others may be admitted to the hospital for closer monitoring in the third trimester.
If blood pressure spikes or labs suggest preeclampsia is getting worse, the medical team may recommend delivering earlyeven if the baby isn’t quite “ready” yet.

Parents who’ve gone through this often talk about the emotional tug-of-war: wanting the baby to stay inside longer to grow, but also wanting to keep everyone safe.
They learn a new vocabulary“biophysical profile,” “Doppler flow,” “growth percentile”and they become experts in their own chart almost overnight.

Expecting one baby… and finding out there are two (or three)

Another common story starts with a routine ultrasound and the tech’s cheerful words: “I see two heartbeats!”
Multiple pregnancy can bring excitement and joy, but it also comes with a higher risk of preterm birth and extra complications.

People carrying twins or triplets often describe feeling like they move onto the “frequent flyer” list at the clinic. There may be more ultrasounds, more weight checks, and stricter instructions about rest and activity.
Sometimes, even with careful monitoring, contractions begin or the water breaks earlier than hoped for, simply because the uterus is under so much stretch and stress.

After delivery, parents of multiples may spend weeks or months visiting the neonatal intensive care unit (NICU), learning to interpret monitors and celebrating every ounce gained and every tube removed.

When there are no obvious risk factors at all

One of the hardest scenarios is when preterm labor starts in someone with no known risk factors: healthy, non-smoker, normal weight, no chronic illness, a straightforward pregnancy so far.
Contractions begin, and despite medications and bed rest, the baby arrives early.

For many families, this can lead to intense feelings of guilt and confusion“What did I miss? What could I have done differently?” The honest answer is often: nothing.
Research shows that many preterm births happen without a clear, identifiable cause, even when prenatal care has been excellent.

Talking with the care team after delivery, asking for a debrief, and sometimes seeking a second opinion or specialist consultation for future pregnancies can help parents understand what’s known and what remains uncertain.
Support groupsboth in-person and onlinecan also play a big role in processing the experience.

Finding support and moving forward

Whether premature birth was linked to infection, chronic illness, multiples, or no clear cause at all, recovery is both physical and emotional.
Parents may juggle postpartum healing with daily NICU visits, pumping schedules, work responsibilities, and caring for older children.

Many people say that what helped most wasn’t any one piece of medical advice, but a combination of:

• Honest, compassionate communication from their care team
• Clear explanations of what was happening and why certain decisions were made
• Connections with other families who’d had premature babies
• Permission to feel everythingfear, grief, hope, joyoften in the same day

Over time, families learn to live with the “what ifs” and focus on the baby in front of them, whether that means navigating oxygen tubing at home, celebrating small developmental milestones, or just enjoying a cuddle without wires for the first time.

Understanding the causes and risk factors for premature birth doesn’t erase the pain of a difficult experience. But it can help parents feel less alone, more informed, and better equipped to advocate for themselves and their babiesboth in this pregnancy and in any pregnancies to come.

Bottom line

Premature birth is common, complex, and often unpredictable. It can be linked to issues with the cervix, uterus, or placenta; infections and inflammation; chronic health conditions; multiple pregnancy; and a wide range of lifestyle and social factors.
Yet sometimes it happens in the absence of any clear risk.

While no strategy can guarantee a full-term birth, there’s a lot you can do to tilt the odds in your favor: get early and consistent prenatal care, manage chronic conditions, avoid smoking and substance use, pay attention to warning signs, and ask questions until you truly understand your options.
Most importantly, remember that if preterm birth happens, it is not your fault. Medicine, technology, and neonatal care have advanced dramatically, and many babies born early can and do go on to thrive.