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What Causes Premature Birth?

Spontaneous Preterm Labor and Medically Necessary Early Births

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Updated April 29, 2013

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Picture of a pregnant woman smoking a cigarette

Smoking can increase your risk of preterm birth.

Image copyright Tim Hawley / Getty Images

Premature birth is the birth of a baby before 37 weeks 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: birth where labor happens on its own, where mom's water breaks early, or where doctors decide that delivering the baby is medically necessary. The first two categories are similar, and can be combined and called "spontaneous preterm birth."

Spontaneous Premature Birth

No matter when it begins, labor is a complicated and often unpredictable series of events. In spontaneous preterm birth, labor starts early and doctors are not able to stop the labor process. Spontaneous preterm labor causes about 2/3 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 preterm premature rupture of membranes, or PPROM for short.

Unfortunately, doctors usually can't tell exactly what caused a mom to go into labor early or to have PPROM. Often there are a combination of risks 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 spontanous 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 or 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 premature 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 6 months apart.
  • Multiple pregnancies: Being pregnant with more than one baby causes the uterus to become overdistended, which can cause labor to start early. The risk of preterm birth from multiple pregnancies increases with the number of babies.
  • 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 black race also increases a woman's chances of early birth.

There are many ways that doctors try to prevent early labor in moms who are at risk. 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.

Read More: Preventing and Stopping Premature Labor

Medically Indicated Preterm Birth

For most women, pregnancy causes only mild discomforts. 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 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 can all cause a baby to have intrauterine growth restriction, or 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 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 to decide when the best time is to deliver the baby. 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.

Sources:

Ananth,C and Vintzileos, A. "Maternal-fetal conditions necessitating a medical intervention resulting in preterm birth." American Journal of Obstetrics and Gynecology. Dec. 2006; 195, 1557–1563.

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.

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