Expectant parents often wonder about pregnancy length and what constitutes a “full term” baby. Understanding gestational age helps clarify when a baby is considered fully developed and ready for birth. This information guides decisions about childbirth timing and helps ensure the healthiest start for a newborn.
Understanding Gestational Age Categories
Medical organizations, including the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM), classify gestational age into precise categories based on completed weeks.
A pregnancy is considered “preterm” if delivery occurs before 37 weeks 0 days. Babies born in this period are at higher risk for various health issues due to their prematurity.
The category of “early term” applies to births occurring from 37 weeks 0 days through 38 weeks 6 days. Thus, a baby born at 38 weeks falls into the early term category, not the premature category.
A pregnancy reaches “full term” from 39 weeks 0 days through 40 weeks 6 days. “Late term” covers 41 weeks 0 days through 41 weeks 6 days, and “post term” refers to pregnancies at 42 weeks 0 days and beyond. These distinctions are important because outcomes can vary significantly even within what was once broadly called “term.”
Key Development Milestones at 38 Weeks
Even at 38 weeks, significant developmental processes are still underway. The final weeks of gestation are important for the maturation of vital organ systems as the baby gains weight and prepares for birth.
The baby’s brain continues its rapid growth and organization during this period. A baby’s brain can increase in size by as much as one-third between 35 and 39 weeks of gestation. This involves continued myelination, where nerve fibers are coated to improve signal transmission, and the formation of new connections between brain cells.
Lung maturation is also progressing, with increasing production of surfactant, a substance that helps the air sacs in the lungs remain open after birth. While some surfactant is present earlier, optimal levels are still developing to ensure stable and independent breathing. The liver is also maturing, which is important for regulating blood sugar and processing bilirubin, a waste product that can cause jaundice if not properly cleared.
The accumulation of subcutaneous fat continues, providing insulation and a vital energy reserve for the newborn. This fat layer helps the baby regulate its body temperature outside the womb. These finishing touches on organ development are still occurring.
Why Waiting Until Full Term Matters
Allowing pregnancy to progress to full term (39 weeks or more) offers significant health advantages for the baby compared to an early term delivery. Waiting a few more weeks can reduce the likelihood of certain complications and improve long-term outcomes.
One primary benefit relates to respiratory health. Babies born at 37 or 38 weeks have a higher risk of breathing problems, such as transient tachypnea of the newborn (TTN), compared to those born at 39 weeks or later. This is because optimal surfactant levels are still developing in the final weeks, which helps prevent the air sacs from collapsing.
Babies delivered early term may also face increased challenges with feeding and maintaining a stable body temperature. Immature suck-swallow-breathe coordination can make breastfeeding more difficult. A less developed layer of subcutaneous fat can impair temperature regulation, potentially leading to hypothermia. The liver’s continued maturation also helps prevent jaundice, as a more developed liver is better equipped to process bilirubin.
Research indicates that full-term birth is associated with improved long-term neurological and developmental outcomes. Studies have shown that infants born at 37 or 38 weeks may have slightly lower cognitive development scores compared to those born at 39 to 41 weeks. These subtle differences can impact learning and behavior later in childhood. Healthcare providers recommend allowing a healthy pregnancy to progress naturally to at least 39 weeks unless there is a medical reason for earlier delivery.