How to use the due date calculator
- Enter the first day of your last period (LMP).
- Adjust your average cycle length if it isn’t 28 days.
- See your estimated due date, how many weeks along you are, your trimester and an estimated conception date.
How due dates are calculated
The standard method is Naegele’s rule: pregnancy lasts about 280 days (40 weeks) from the first day of your last period. Because dating starts from your period rather than conception, you’re considered “2 weeks pregnant” around the time you actually conceive.
conception ≈ last period + (cycle − 14)
The three trimesters
| Trimester | Weeks |
|---|---|
| First trimester | 0 – 13 |
| Second trimester | 14 – 27 |
| Third trimester | 28 – 40+ |
Frequently asked questions
Probably not. Only about 1 in 20 babies are born on their due date. Most arrive within two weeks before or after, which is considered full term.
The last period is a date most people know, while the exact moment of conception is hard to pinpoint. Dating from the LMP gives a consistent 40-week standard.
Enter your real average. A longer cycle pushes the due date later and a shorter one brings it earlier, because ovulation shifts accordingly.
A first-trimester dating ultrasound is more accurate, especially with irregular cycles. Use this as a friendly estimate between appointments.