The journey of pregnancy is often measured by the anticipated 40-week benchmark, which represents the average duration of a full-term gestation. Gestational age is the timeline medical professionals use to monitor fetal development. While 40 weeks is the general expectation, birth timing varies significantly. The 37-week milestone is particularly noteworthy because it historically marked the transition from a premature birth to a term birth.
Defining the 37-Week Milestone
A baby born at the start of the 37th week has officially completed 37 full weeks of gestation and is no longer categorized as a preterm birth. Preterm birth is clinically defined as any delivery occurring before 37 weeks (36 weeks and six days or earlier). Health risks associated with delivery decline sharply once the 37-week mark is reached.
The percentage of births occurring at exactly 37 weeks is a significant portion of all deliveries. Based on provisional data for singleton births in the United States, approximately 12.05% of babies were born at 37 weeks gestation as of 2023. This figure represents a notable increase in recent years, partly influenced by a rise in medically indicated and non-medically indicated inductions of labor.
The Importance of Gestational Classification
While a baby born at 37 weeks is no longer classified as preterm, major medical organizations now use more nuanced terminology to define the final weeks of pregnancy. Organizations like the American College of Obstetricians and Gynecologists (ACOG) established new categories to reflect differences in newborn health outcomes. Previously, any birth between 37 and 42 weeks was simply called “term,” but research showed outcomes were not uniform across this period.
The current classification system defines the period from 37 weeks 0 days through 38 weeks 6 days as Early Term. This designation acknowledges that while these babies are generally healthy, they may not be as developmentally mature as those born later. The optimal window for delivery, associated with the best health outcomes, is designated as Full Term, which spans from 39 weeks 0 days through 40 weeks 6 days.
The period just before the 37-week boundary is known as Late Preterm, covering births from 34 weeks 0 days through 36 weeks 6 days. The updated definitions emphasize that every week of gestation is meaningful, particularly for the maturation of the brain, lungs, and liver. These classifications encourage healthcare providers to avoid elective deliveries before 39 weeks unless a medical necessity exists.
Health Considerations for Babies Born at 37 Weeks
Although a baby born at 37 weeks is considered Early Term, they may face different short-term challenges compared to infants born at 39 or 40 weeks. Delivery at 37 weeks can mean that some systems are not completely ready for life outside the uterus, as the final weeks in the womb allow for important developmental processes. One common issue is difficulty regulating body temperature, as the baby may have less subcutaneous fat to maintain warmth compared to a Full Term infant.
Babies born at this stage are also at an increased risk for transient tachypnea of the newborn (TTN), a mild respiratory condition characterized by rapid breathing shortly after birth. This is often due to a delay in clearing fetal lung fluid, a process that is typically aided by the hormonal changes of labor. Many Early Term infants experience feeding difficulties because the coordination of sucking, swallowing, and breathing may not be fully developed.
The risk of developing neonatal jaundice, which often requires phototherapy treatment, is also elevated in 37-week infants. These newborns are more likely to require admission to the neonatal intensive care unit (NICU) compared to those born at 39 or 40 weeks. This increased risk of short-term complications underscores why medical guidelines promote waiting until the Full Term period for delivery whenever possible.