What is preterm labor and birth?
In general, a normal human pregnancy is about 40 weeks long (9.2 months). Health care providers now define “full-term” birth as birth that occurs between 39 weeks and 40 weeks and 6 days of pregnancy.1 Infants born during this time are considered full-term infants.
Infants born in the 37th and 38th weeks of pregnancy—previously called term but now referred to as “early term”—face more health risks than do those born at 39 or 40 weeks.2
Deliveries before 37 weeks of pregnancy are considered “preterm” or premature:
- Labor that begins before 37 weeks of pregnancy is preterm or premature labor.
- A birth that occurs before 37 weeks of pregnancy is a preterm or premature birth.
- An infant born before 37 weeks in the womb is a preterm or premature infant. (These infants are commonly called “preemies” as a reference to being born prematurely.)
“Late preterm” refers to 34 weeks through 36 weeks of pregnancy. Infants born during this time are considered late-preterm infants, but they face many of the same health challenges as preterm infants. More than 70% of preterm infants are born during the late-preterm time frame.3
Preterm birth is the most common cause of infant death and is the leading cause of long-term disability in children.4 Many organs, including the brain, lungs, and liver, are still developing in the final weeks of pregnancy. The earlier the delivery, the higher the risk of serious disability or death.
Infants born prematurely are at risk for cerebral palsy (a group of nervous system disorders that affect control of movement and posture and limit activity), developmental delays, and vision and hearing problems.
Late-preterm infants typically have better health outcomes than those born earlier, but they are still three times more likely to die in the first year of life than are full-term infants.3 Preterm births can also take a heavy emotional and economic toll on families.5
American College of Obstetricians and Gynecologists. (2013). Definition of term pregnancy. Committee Opinion No. 579.
Obstetrics and Gynecology, 122, 1139–1140.
Spong, C. Y. (2013). Defining “term” pregnancy: recommendations from the Defining “Term” Pregnancy Workgroup.
Journal of the American Medical Association, 309, 2445–2446.
March of Dimes. (2011).
Prematurity research. Retrieved September 17, 2013, from
Centers for Disease Control and Prevention. (n.d.).
Preterm birth. Retrieved September 17, 2013, from
March of Dimes. (2012).
The March of Dimes Foundation Data Book for Policy Makers: Maternal, Infant, and Child Health in the United States 2012. Retrieved March 5, 2014, from
http://www.marchofdimes.com/materials/Databookforpolicymakers.pdf (PDF – 10.1 MB)