How Pregnancy Due Date Is Calculated
Naegele's Rule
Due Date = LMP + 280 days (40 weeks)
Adjustment: If your cycle is longer than 28 days, add the difference. For a 30-day cycle, add 2 days to the due date.
Pregnancy Trimesters
| Trimester | Weeks | Key Milestones |
|---|---|---|
| First | 1-13 | Organ formation, heartbeat detectable |
| Second | 14-27 | Movement felt, gender visible on scan |
| Third | 28-40 | Rapid growth, lungs mature |
How to Use This Pregnancy Calculator
- Enter the first day of your last period.
- Enter your average cycle length (default 28 days).
- See your due date, current week, and trimester instantly.
Frequently Asked Questions
How is pregnancy due date calculated?
Add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is Naegele's rule.
How many weeks is a full-term pregnancy?
Full-term is 39-40 weeks. 37-42 weeks is normal range.
What are the three trimesters?
1st: weeks 1-13. 2nd: weeks 14-27. 3rd: weeks 28-40.
Can the due date change?
Yes. Early ultrasound (8-12 weeks) is most accurate and may adjust the due date.
What is conception date?
Conception usually happens ~2 weeks after your last period, around ovulation.