The due date (EDD) is an estimate — only about 4% of babies are born on their exact due date. Most are born within ±2 weeks.
Naegele's Rule
EDD = LMP + 1 year − 3 months + 7 days. Example: LMP = March 1 → EDD = December 8.
Trimester Breakdown
- 1st trimester: Weeks 1–12 (organ development)
- 2nd trimester: Weeks 13–26 (growth, movement)
- 3rd trimester: Weeks 27–40 (weight gain, preparation for birth)
Ultrasound Dating vs. LMP Dating
First-trimester ultrasound measurements (crown-rump length) are considered more accurate than last-menstrual-period-based dating, especially for people with irregular cycles, and often adjust the estimated due date by several days. Once set by early ultrasound, the due date is typically not changed by later scans.
Full Term Isn't a Single Day
"Full term" now refers to a range: early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), and post-term (42+ weeks) — reflecting research showing outcomes vary across this window, not just before vs. after exactly 40 weeks.