How this due date calculator works
A pregnancy is conventionally measured from the first day of your last menstrual period (LMP), not from conception. That is because the LMP is a date most people can identify, while the exact moment of conception is rarely known. By this convention, "weeks pregnant" includes roughly the two weeks before you actually conceived.
The standard estimate uses Naegele's rule: add 280 days — that is 40 weeks — to the first day of the LMP. If your menstrual cycle is longer or shorter than the typical 28 days, the calculator shifts the due date by the difference, since ovulation (and therefore conception) is offset accordingly. If you instead know your conception date, the tool adds about 266 days (38 weeks), because conception happens roughly two weeks after the LMP.
| Milestone | Gestational age | What it marks |
|---|---|---|
| First trimester | Weeks 1–13 | LMP through end of week 13 |
| Second trimester starts | Week 14 | LMP + 14 weeks (98 days) |
| Third trimester starts | Week 28 | LMP + 28 weeks (196 days) |
| Full term begins | Week 37 | LMP + 37 weeks (259 days) |
| Estimated due date | Week 40 | LMP + 40 weeks (280 days) |
Reference note: calculations use your device's local date and a 40-week (280-day) convention. A due date is an estimate, and a clinical due date set by ultrasound may differ. Your healthcare provider's date takes precedence.
Frequently asked questions
- How is a due date calculated?
- The most common method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), known as Naegele's rule. If you know your conception date instead, the estimate is about 266 days from conception. A longer or shorter menstrual cycle shifts the date by the difference from a 28-day cycle.
- What is Naegele's rule?
- Naegele's rule estimates the due date by taking the first day of the last menstrual period, subtracting three months and adding seven days and one year. In practice this equals about 280 days, or 40 weeks, after the LMP.
- How many weeks is a full-term pregnancy?
- A pregnancy is counted as about 40 weeks from the last menstrual period. A birth is considered full term between 37 and 42 weeks; before 37 weeks is preterm and after 42 weeks is post-term.
- How accurate is the due date?
- A due date is only an estimate. Only about 5 percent — roughly 1 in 20 — of babies arrive on their exact due date. Most births happen within a couple of weeks on either side, which is why providers describe a due window rather than a single day.
- How do I calculate due date from conception?
- If you know the date of conception, add about 266 days (38 weeks) to estimate the due date. Conception typically occurs around two weeks after the first day of the last menstrual period, which is why the conception-based count is about 14 days shorter than the LMP-based count.
- Is this medical advice?
- No. This calculator is for general information and education only and is not medical advice. Estimates can differ from a clinical due date set by ultrasound. Always rely on your healthcare provider's assessment, and their date takes precedence.