How Your Due Date Is Calculated
This calculator uses Naegele's Rule, the same method most healthcare providers start with: the estimated due date is 280 days (40 weeks) after the first day of your last menstrual period (LMP). Pregnancy weeks are counted from the LMP too, which is why "week 6" often arrives just two weeks after a positive test.
Due date = LMP + 280 days
The Trimesters
| Trimester | Weeks | Highlights |
|---|---|---|
| First | 1–12 | Organ formation, first heartbeat, morning sickness peaks |
| Second | 13–27 | Anatomy scan (~20 w), first movements, energy returns |
| Third | 28–40 | Rapid growth, lung maturation, birth preparation |
How Accurate Is the Estimate?
Only about 5 % of babies arrive on their exact due date; most are born within two weeks either side. Naegele's Rule assumes a regular 28-day cycle with ovulation on day 14 — if your cycles differ, the first-trimester ultrasound (crown-rump length) gives the more accurate dating your provider will use.
This tool is for orientation only and does not replace prenatal care.