(CNN) -- In light of recent research showing that each week of gestation affects newborn health, a leading doctors' group is redefining when a pregnancy is considered "term."
Previously, a pregnancy was considered full term any time between 37 and 42 weeks of gestation. On average, a single-fetus pregnancy lasts 40 weeks from the date of the woman's last menstrual period.
In a joint statement, the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine said the change is being made "to improve newborn outcomes and expand efforts to prevent nonmedically indicated deliveries before 39 weeks of gestation."
Under the new definitions, the period between 37 weeks and 38 weeks, six days of gestation is considered "early term." Thirty-nine weeks to 40 weeks, six days is considered "full term"; between 41 weeks 0 days and 41 weeks, six days is "late term"; and 42 weeks and beyond is considered "postterm."
Research over the past several years has shown that babies' brain and lungs fully mature during the last few weeks of pregnancy. Babies born between 39 weeks and 40 weeks, six days have the best health outcomes, the American College of Obstetricians and Gynecologists said in a statement.
"This terminology change makes it clear to both patients and doctors that newborn outcomes are not uniform even after 37 weeks," Dr. Jeffrey Ecker, chair of the group's Committee on Obstetric Practice, said in the statement. "Each week of gestation up to 39 weeks is important for a fetus to fully develop before delivery and have a healthy start."
Planned deliveries before 39 weeks should only occur when continuing a pregnancy poses "significant health risks" to a mother or baby, the group said, noting that sometimes delivery before then is unavoidable, such as when a women's water breaks or contractions come early.