How Your Due Date Is Calculated
Naegele's Rule, developed in the 19th century, is the standard method: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles, the conception date or ultrasound dating may be more accurate.
Ultrasound dating, especially in the first trimester (before 12 weeks), is highly accurate — within 3–5 days. Later ultrasounds have wider margins of error. If ultrasound and LMP dates disagree by more than 7 days, providers typically adjust the due date to the ultrasound estimate.
Due Date Formulas
Naegele's Rule: EDD = LMP + 280 days
Conception-based: EDD = Conception Date + 266 days
Ultrasound: EDD = Ultrasound Date + (280 − Weeks×7) days
Pregnancy Timeline Overview
| Period | Weeks | Key Development |
|---|---|---|
| 1st Trimester | 1–12 | Organ formation, embryo becomes fetus |
| 2nd Trimester | 13–26 | Growth, movement felt, anatomy scan |
| 3rd Trimester | 27–40 | Rapid growth, lung development, birth prep |
| Full Term | 37–42 | Baby considered full term, delivery expected |
Frequently Asked Questions
Only about 5% of babies arrive on their exact due date. Most births occur within 2 weeks of the EDD. Providers consider 37–42 weeks as normal full-term range. The due date is best thought of as the midpoint of a 5-week delivery window rather than a precise target.
Irregular cycles make LMP-based dating less accurate. Conception date (if known from ovulation tracking or IVF) or early ultrasound provides better accuracy. First-trimester ultrasound (8–12 weeks) is the most reliable way to establish gestational age when cycles are irregular.
Gestational age counts from the first day of the last menstrual period (about 2 weeks before conception). Fetal age (post-conceptional age) counts from conception. Doctors always use gestational age, so when they say '12 weeks pregnant,' the fetus is actually about 10 weeks old.
Contact your provider at 40 weeks if you haven't already. Most providers monitor closely from 40–41 weeks. At 41–42 weeks, induction is typically offered or recommended, as the risk of complications increases slightly after 42 weeks. Never wait more than 42 weeks without medical evaluation.