Definitions of Early Preterm, Late Preterm, and Term Pregnancy
Late preterm pregnancy is defined as 34 0/7 through 36 6/7 weeks of gestation, early term is 37 0/7 through 38 6/7 weeks, and full term begins at 39 0/7 weeks. 1, 2
Gestational Age Framework
Gestational age (GA) is calculated from the first day of the last menstrual period or by early-dating ultrasound examination, and is reported as completed weeks and days (e.g., 34 0/7 means 34 weeks plus 0 days). 3, 4 The first trimester extends through 13 weeks 6 days GA. 3, 4
Preterm Birth Categories
Late Preterm (34 0/7 - 36 6/7 weeks)
- This represents the largest subgroup of preterm births, accounting for approximately 3-6% of singleton live births internationally. 5, 2
- These infants are physiologically immature with limited compensatory responses to extrauterine life compared to term infants, despite often appearing similar in size and weight. 2
- Late preterm infants have 2-3 fold increased risk of hypothermia, hypoglycemia, respiratory distress, feeding difficulties, jaundice, sepsis, and hospital readmission. 2
Early Preterm (< 34 weeks)
- While not explicitly defined in the provided guidelines, this category encompasses births occurring before 34 0/7 weeks of gestation. 6
- These infants face substantially higher morbidity and mortality risks than late preterm infants. 6
Term Birth Categories
Early Term (37 0/7 - 38 6/7 weeks)
- This subcategory was established after recognizing that "term" is not a homogeneous group. 7, 1
- Early term births represent 15-31% of singleton live births across different countries. 5
- Infants born at early term have significantly increased mortality and neonatal morbidity compared to those born at 39 weeks or later. 7, 1
Full Term (39 0/7 - 40 6/7 weeks)
- This represents the optimal window for delivery in uncomplicated pregnancies. 7
- Birth at 39 weeks or later is associated with the best child health outcomes across diverse settings. 5
Late Term (41 0/7 - 41 6/7 weeks)
- Pregnancies extending beyond 40 weeks but before 42 weeks. 7
Postterm (≥ 42 0/7 weeks)
- Pregnancies continuing to or beyond 42 completed weeks of gestation. 7
Clinical Implications
The designation "term" carries critical implications for pregnancy management, timing of elective delivery, and management of complications. 7 The historical definition of term (37-41 weeks) was determined somewhat arbitrarily and fails to account for significant outcome differences within this 5-week interval. 7
Key Clinical Pitfalls to Avoid
- Do not treat late preterm infants (34-36 weeks) as "near-term" or functionally mature—they require heightened surveillance and should not routinely be managed in well-baby nurseries. 2
- Avoid elective delivery before 39 weeks in uncomplicated pregnancies, as this increases neonatal morbidity. 1
- For medically indicated deliveries between 34-38 weeks, the timing must balance maternal/fetal risks of continued pregnancy against prematurity-related complications. 6, 1
- Amniocentesis for fetal lung maturity in well-dated pregnancies should generally not guide delivery timing in the late preterm or early term period. 1