Causes of Premature Birth

A premature baby in the hospital

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Premature birth is the birth of a baby before 37 weeks of gestation. There are many factors that may combine to cause early birth, and it's not always possible to say exactly what caused a pregnancy to end early.

Causes of premature birth can be split into main three categories: when labor happens on its own, when mom's water breaks early, and when doctors decide that delivering the baby is medically necessary. The first two categories are similar and can be combined and called "spontaneous preterm birth." Take a closer look at the categories, below. 

Spontaneous Premature Birth

No matter when it begins, labor is a complicated and often unpredictable series of events. In spontaneous premature birth, labor starts early and doctors are not able to stop the labor process. Spontaneous preterm labor causes about two-thirds of all premature births.

In spontaneous premature birth, labor can start either with typical labor contractions or with mom's water breaking. If mom's water breaks before 37 weeks, it's called premature rupture of membranes, or PPROM for short.

Unfortunately, doctors usually can't tell exactly what caused a mom to go into preterm labor or to have PPROM. Often, various risks are present. Doctors do know that the following risks greatly increase a mom's chance of PPROM or premature labor:

  • Infection: Some type of infection is present in many cases of spontaneous premature birth. Any kind of systemic inflammation or infection can cause a mom to have her baby early, including infections in the mouth (such as gum disease), vagina, uterus, and kidneys.
  • Cervical Problems: Insufficient cervix or short cervix both increase the risk of preterm birth, especially if the mom is having labor symptoms.
  • Smoking: Any kind of tobacco use increases a mom's risk of PPROM and preterm labor. Nicotine causes blood vessels in the uterus to constrict, which can prevent nutrients and oxygen from getting to the baby or contribute to early labor. 
  • Stress: Chronic, high-level psychological stress can cause labor to start early.
  • Short Time Between Pregnancies: The risk of preterm birth is two times higher than normal if pregnancies are less than six months apart.
  • Carrying Twins, Triplets, and More: Being pregnant with more than one baby causes the uterus to become overdistended, which can cause labor to start early. The more babies you are carrying, the higher your risk of preterm labor. 
  • Genetics: Your risk of delivering early is higher if your mom or sister went into labor early, or if you've had a previous premature baby. Doctors aren't sure why, but being Black also increases a woman's chances of an early birth.

There are many ways that doctors try to prevent early labor in at-risk moms. If you are at risk for preterm birth, you'll be followed closely by your doctor and you may need to see a specialist who works with high-risk pregnancies. 

Medically Indicated Preterm Birth

For most women, pregnancy causes only mild discomfort. In some women, though, pregnancy causes severe health problems that can threaten the lives of mom and baby. In these cases, doctors may decide to deliver the baby early—even if mom is not in labor. Some of the most common medical reasons why a baby may be born early include:

  • Preeclampsia: Preeclampsia is a life-threatening condition that causes high blood pressure and protein in the urine. In severe cases, it can cause seizures or be fatal. Medications can help, but the delivery of the baby is the only cure for preeclampsia.
  • Poor Fetal Growth: There can be many reasons why a baby is not growing well inside of mom. Problems with the placenta, certain infections, twin gestation, or genetic anomalies in the baby all can cause a baby to have intrauterine growth restriction (IUGR). In some cases, the baby may need to be delivered early.
  • Placental Abruption: In some pregnancies, the placenta starts to separate from the uterus before the baby is born. Called placental abruption, this can cause extreme blood loss in mom and baby and can be fatal. Emergency delivery of the baby is necessary.
  • Fetal Distress: Sometimes a baby is in distress during a pregnancy for reasons that may not be known. Problems with the cord, problems with blood flow, and maternal liver disease are a few causes of fetal distress.

Some medically indicated premature births are emergency births where the decision to deliver must be made very quickly. Others are due to more chronic conditions where doctors watch mom and baby very closely over time to decide when the best time is to deliver the baby.

A Word From Verywell

By following your doctor's orders and working with a doctor with whom you have a close and trusting relationship, you'll know for sure that you're doing the very best for yourself and your baby.

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3 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Loftin RW, Habli M, Snyder CC, Cormier CM, Lewis DF, Defranco EA. Late preterm birth. Rev Obstet Gynecol. 2010;3(1):10-9.

  2. Suman V, Luther EE. Preterm Labor. Treasure Island, FL: StatPearls Publishing.

  3. Ananth CV, Vintzileos AM. Medically indicated preterm birth: recognizing the importance of the problem. Clin Perinatol. 2008;35(1):53-67, viii. doi:10.1016/j.clp.2007.11.001

Additional Reading
  • Goldenberg, R., Culhane, J., Iams, J., and Romero, R. "Epidemiology and Causes of Preterm Birth." The Lancet. Jan. 2008; 371, 74-83.

  • Voltolini, C. et al. "Understanding Spontaneous Preterm Birth: From Underlying Mechanisms to Predictive and Preventive Interventions." Reproductive Sciences March 2013.

By Cheryl Bird, RN, BSN
Cheryl Bird, RN, BSN, is a registered nurse in a tertiary level neonatal intensive care unit at Mary Washington Hospital in Fredericksburg, Virginia.