Full term vs early term vs preterm: what the weeks mean

What does the evidence say about pregnancy term definitions during pregnancy?

1. The term-pregnancy consensus defines early term as 37 weeks through 38 weeks and 6 days, full term as 39 weeks through 40 weeks and 6 days, late term as 41 weeks through 41 weeks and 6 days, and postterm as 42 weeks or later. 1
2. NICHD describes preterm birth as birth before 37 completed weeks of pregnancy. 2
3. A cohort study found higher short-term neonatal morbidity among early-term infants than infants born at 39 to 41 weeks, showing why the labels are not interchangeable. 3
4. A large study found neonatal intensive-care admission and respiratory morbidity were lowest at or beyond 39 weeks for most delivery precursors, while decisions about earlier delivery can still be medically indicated. 4
5. NICHD notes that preterm birth has different causes and does not always begin with preterm labor, so a timing label is not an explanation for why birth occurred. 2

Is pregnancy term definitions safe in each trimester?

• First trimester. Pregnancy term labels are assigned from gestational age, usually dated from the first day of the last menstrual period and refined by early ultrasound when appropriate.
• Second trimester. The second trimester is a useful time to confirm how your due date and gestational weeks are being counted in the prenatal record.
• Third trimester. Term labels become clinically relevant in the third trimester, but they do not by themselves decide when delivery should occur.