ACOG's term categories
ACOG classifies births from 37 weeks of gestation into four categories, counted from the first day of the LMP:1
| Category | Weeks of gestation |
|---|---|
| Early term | 37 weeks 0 days through 38 weeks 6 days |
| Full term | 39 weeks 0 days through 40 weeks 6 days |
| Late term | 41 weeks 0 days through 41 weeks 6 days |
| Postterm | 42 weeks 0 days and beyond |
Before 37 weeks, a birth is preterm: the World Health Organization defines preterm as babies born alive before 37 weeks of pregnancy are completed.2
Weeks here are completed weeks and days, so "38 weeks 6 days" is the last day of early term. How pregnancy weeks are counted explains the notation.
Why "term" was split into categories
In the past, the period from 3 weeks before until 2 weeks after the estimated due date was called "term", on the expectation that outcomes for babies born in that window were uniform and good. Research has shown that outcomes, especially breathing problems (respiratory morbidity), vary depending on when in that 5-week range the baby is born.1
A work group convened in late 2012 therefore recommended replacing the single label with early term, full term, late term and postterm, and discouraged using "term" for all births between 37 weeks 0 days and 41 weeks 6 days.1
How the categories line up with your due date
Pregnancy lasts an average of 40 weeks (280 days) from the first day of the LMP to the estimated due date.1 The due date itself, 40 weeks 0 days, falls inside the full-term range, a week after it starts. Counted in days from the due date, the categories are:
| Category | Relative to the due date | Example dates |
|---|---|---|
| Early term | 21 to 8 days before | April 17 to April 30, 2027 |
| Full term | 7 days before to 6 days after | May 1 to May 14, 2027 |
| Late term | 7 to 13 days after | May 15 to May 21, 2027 |
| Postterm | 14 days after or later | From May 22, 2027 |
Births spread across several weeks around that date: most babies are born between 37 and 42 weeks of pregnancy.3 In a study of 34,249 pregnancies, 49.5% of births happened within 7 days either side of a due date estimated from the LMP.4
The categories are only as accurate as the due date they are counted from. A first-trimester ultrasound is the most accurate way to establish or confirm gestational age,5 so a due date confirmed or set by an early scan is the more accurate reference; see ultrasound dating vs. LMP dating.
Terms you may see elsewhere
The World Health Organization's ICD-11 Reference Guide uses broader categories: term is from 37 completed weeks to less than 42 completed weeks (259 to 293 days), and post-term is 42 completed weeks or more (294 days or more).6 ACOG's early, full and late term are subdivisions of that same 37-to-42-week span.
Labels, not a plan
These categories describe when a birth happens. They are not a recommendation for any pregnancy. Questions about the timing of your own birth are for your doctor or midwife.
Find your 40-week date
Enter the first day of your last period and the Due Date Calculator shows the estimated due date the term categories are counted around.
Common questions
Is 37 weeks full term?
Not in ACOG's categories: 37 weeks 0 days to 38 weeks 6 days is early term, and full term starts at 39 weeks 0 days.1 Under the broader WHO definition, 37 completed weeks is the start of term.6
Is 40 weeks full term?
Yes. 40 weeks 0 days is the estimated due date, inside the full-term range of 39 weeks 0 days to 40 weeks 6 days.1
What does postterm mean?
A pregnancy at 42 weeks 0 days or later, two weeks or more past the estimated due date.1
Sources
- American College of Obstetricians and Gynecologists (ACOG). Definition of Term Pregnancy (Committee Opinion No. 579). 2013; reaffirmed 2025.
- World Health Organization (WHO). Preterm birth (fact sheet). 2023.
- Pregnancy, Birth and Baby (Healthdirect Australia, funded by the Australian Government). Working out your due date. reviewed August 2024.
- American Journal of Obstetrics and Gynecology. Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. 174(1 Pt 1):278–81. 1996.
- American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
- World Health Organization (WHO). ICD-11 Reference Guide. accessed October 2026.