Preterm Birth: Risks, Prevention and Outcomes

What counts as preterm birth

A full-term pregnancy lasts about 40 weeks. Babies born before 37 weeks are preterm:

About two-thirds of preterm births are spontaneous, following preterm labor or early rupture of the waters. The rest are medically indicated, meaning doctors recommend early delivery because continuing the pregnancy is riskier for mother or baby, for example in severe preeclampsia or poor fetal growth.

Risk factors

Strongest predictors

• A previous spontaneous preterm birth, especially at an early gestation. The earlier and more often it happened, the higher the risk next time.
• A short cervix on transvaginal ultrasound in mid-pregnancy, typically 25 mm or less before 24 weeks.
• Twins, triplets or more (multiple pregnancy). More than half of twins are born preterm.

Pregnancy complications

• preeclampsia and high blood pressure
• placental abruption and placenta previa
• premature rupture of the membranes
• infections of the uterus, urinary tract or vagina, including bacterial vaginosis in some studies
• vaginal bleeding in more than one trimester
• too much or too little amniotic fluid
• birth defects in the baby

Mother's health and history

• cervical insufficiency, previous cervical surgery (cone biopsy or LEEP) or uterine abnormalities
• chronic high blood pressure, diabetes, kidney disease and autoimmune disease
• being underweight or obese
• age under 17 or over 35
• pregnancies less than 18 months apart
• IVF pregnancy

Lifestyle and social factors

• smoking, vaping, alcohol and drug use
• high stress, intimate partner violence, long working hours on your feet
• limited access to prenatal care

In the US, Black women have notably higher preterm birth rates than white women, a gap linked to differences in access to care, chronic stress and structural racism rather than genetics.

Many preterm births occur in people with no known risk factor.

How preterm birth is predicted

• History: your obstetric provider will ask about earlier births, losses and cervical surgery.
• Cervical length screening: a transvaginal ultrasound measures the cervix. ACOG recommends serial checks from about 16 to 24 weeks for people with a previous spontaneous preterm birth, and many practices measure it at the anatomy scan around 18 to 22 weeks for everyone.
• Fetal fibronectin test: a swab of vaginal fluid in someone with symptoms. A negative result means preterm birth in the next 1 to 2 weeks is unlikely.

How to lower the risk of preterm birth

Medical measures

• Vaginal progesterone: for people with a short cervix (25 mm or less) carrying one baby, nightly vaginal progesterone lowers the risk of preterm birth. Injectable 17-OHP caproate (Makena) was withdrawn from the US market in 2023 after a large trial showed no benefit.
• Cervical cerclage: a stitch placed around the cervix. It is offered for a history of cervical insufficiency, or a short cervix in someone with a previous preterm birth.
• Low-dose aspirin: recommended from 12 to 28 weeks (ideally before 16) for people at high risk of preeclampsia. It lowers the risk of preeclampsia and the preterm births it causes.
• Treating urinary infections found on screening, even without symptoms.
• Managing chronic conditions such as high blood pressure and diabetes before and during pregnancy.
• Single embryo transfer in IVF reduces twin pregnancies.

What you can do

• Start prenatal care early and keep appointments.
• Stop smoking and vaping, and avoid alcohol and drugs. Help is available through 1-800-QUIT-NOW.
• Aim for at least 18 months between giving birth and the next pregnancy.
• Reach a healthy weight before pregnancy if possible.
• Learn the warning signs of preterm labor.

Bed rest and avoiding sex have not been shown to prevent preterm birth and are no longer routinely recommended. Bed rest increases the risk of blood clots and muscle loss.

When preterm birth is likely

If preterm delivery looks likely, care focuses on protecting the baby:

• Corticosteroid injections (betamethasone or dexamethasone) for the mother between 24 and 34 weeks, and in selected cases up to 36 weeks, speed up the baby's lung development and reduce brain bleeding.
• Magnesium sulfate before 32 weeks lowers the risk of cerebral palsy.
• Antibiotics for broken waters or group B strep.
• Tocolytics, medicines that slow contractions, may delay birth by up to 48 hours so steroids can work and the mother can be moved to a hospital with a suitable NICU.

Outcomes for babies

Every extra week in the womb matters. Survival and long-term health improve steadily with gestational age:

• Babies born at 22 to 23 weeks face high risks, and survival depends greatly on the hospital and on whether active treatment is offered.
• By 28 weeks, most babies survive with NICU care.
• Late preterm babies usually do well but are more likely than term babies to need help with breathing, feeding, jaundice and temperature control.

Complications of prematurity include respiratory distress syndrome, brain bleeding, infections, bowel inflammation (necrotizing enterocolitis), eye problems (retinopathy of prematurity) and later developmental, learning, vision and hearing problems. More detail on the baby's care is on our page about premature birth.

Your health after a preterm birth

A preterm birth, especially with preeclampsia, is linked to a higher lifetime risk of heart disease and high blood pressure for the mother. Tell your primary care doctor and keep up regular blood pressure, cholesterol and blood sugar checks. In a future pregnancy, see your provider early so prevention can start on time.

Frequently asked questions

What is the most common cause of preterm birth?

Most preterm births follow spontaneous preterm labor or early rupture of the membranes, often without a single clear cause. Inflammation or infection, overstretching of the uterus with twins, bleeding from the placenta and cervical problems all contribute. About a third are planned early deliveries for medical reasons such as preeclampsia.

Can preterm birth be prevented?

Some can. Vaginal progesterone for a short cervix, cerclage for cervical insufficiency, low-dose aspirin for people at high risk of preeclampsia, stopping smoking and spacing pregnancies at least 18 months apart lower the risk. Many preterm births still occur in people without any known risk factor.

If I had one preterm baby, will the next one be early too?

Your risk is higher than average, but most people with a previous preterm birth deliver at term next time. Tell your provider early in pregnancy. They will usually offer serial cervical length scans from around 16 weeks and treatments such as progesterone or cerclage if the cervix shortens.

What is the difference between preterm birth and premature birth?

They mean the same thing: birth before 37 completed weeks of pregnancy. "Preterm" is the term preferred by doctors and researchers. "Premature" is common in everyday use and also describes the baby, as in a premature or "preemie" baby.

How early can a baby be born and survive?

Some babies survive from about 22 to 23 weeks with intensive care, but risks of death and serious disability are high at that point. Survival rises quickly with each week, and most babies born after 28 weeks survive. Decisions near the edge of viability are made together with the family and the neonatal team.