Premature Birth 101

Premature birth—also called preterm birth—typically occurs when a baby is born three or more weeks before their due date, or before 37 weeks’ gestation. In the United States, 10% of babies are born prematurely.1

While not all premature babies face complications, those born earlier tend to experience more severe medical problems than those born later. Prematurity is also the leading cause of newborn death in the United States.

newborn inside of incubator
doctor showing pregnant women her clipboard

There Are Two Main Types of Preterm Birth:

  • Spontaneous preterm birth:  A preterm birth that occurs on its own because a mother goes into labor or the water breaks
  • Medically-indicated preterm birth: A preterm birth that is induced by a doctor because of risks to the mom or baby if the pregnancy continues

Each Day Matters

In the last few weeks in the womb, your baby experiences critical growth and development that allows them to survive and thrive in the world. In the weeks leading up to the due date, baby’s brain, eyes, and lungs, among other systems, will become developed enough to allow them to make the best start possible. 

In the womb:

  • Week 30: Your baby’s retinas are developed enough to sense light
  • Week 32: Your baby begins to regulate their own body temperature
  • Week 34: Your baby begins to further develop their muscles and intestines
  • Week 36: Your baby’s lungs have all the structures in place to function outside the womb

Though modern neonatal intensive care units (NICUs) are experienced in nurturing and treating preemies, (babies born before 37 weeks of pregnancy), experts agree that the womb is nature’s preferred place for babies to develop until they reach full term. Babies whose time in the womb is shortened by preterm birth face higher risks of problems caused by prematurity.


Fetal development by week

Infant Death

In the United States, about one third of newborn deaths are caused by prematurity. Infants born very early – born before they reach 32 weeks of gestation – have higher mortality rates. Babies who are born closer to their due date have higher chances of survival.

Short and Long-term Health Problems

Preterm babies tend to have higher rates of health problems, such as cerebral palsy, intellectual disabilities, and problems with vision and hearing.

The Benefits of Knowing Your Risk of Preterm Birth

When a pregnancy is at higher risk of ending early, there are steps doctors and moms can take to improve a baby’s chance of surviving and being healthy.

Until recently, physicians have had limited ways of trying to predict each woman’s risk of preterm birth. The known signs — such as a shortened cervix measured during an ultrasound or a mother’s history of a previous preterm delivery – could predict only a small fraction of the preterm births that eventually occur. For most mothers, spontaneous premature birth often is a completely unexpected event.

Sera Prognostics has developed an innovative test that measures the levels of certain proteins in the blood during weeks 18 through 20 to provide an individualized risk assessment for spontaneous preterm birth. Rather than providing a “yes” or “no” answer to whether you will deliver early, the PreTRM Test provides key individualized information about your risk of preterm delivery.

With the PreTRM Test, you and your doctor can better understand your own risk for preterm delivery and work together to develop a treatment plan to manage these risks.

Quick Fact

PreTRM is an innovative test that measures the levels of certain proteins in the blood during weeks 18 through 20 to provide an individualized risk assessment for spontaneous preterm birth for most singleton pregnancies.

Learn More

Talk to your doctor today about the PreTRM Test for your individual risk assessment.

Group 8


  1. Hamilton BE, et al. Births: Provisional data for 2020. Vital Statistics Rapid Release; no 12. Hyattsville, MD: National Center for Health Statistics. May 2021.