Pregnancy Trimester Calculator
Find out which trimester you're in from your last menstrual period.
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Your result
2
Trimester
AI explanation
Formula
Trimester 1: weeks 0-12. Trimester 2: weeks 13-26. Trimester 3: weeks 27+Worked example
LMP 2026-01-01, 15 weeks later
| Field | Value |
|---|---|
| First day of last menstrual period | 01-01-2026 |
| As of date | 16-04-2026 |
| Trimester | 2 |
Assumptions
- Trimesters are divided at week 13 (end of first trimester) and week 27 (end of second trimester), a common convention — some sources use slightly different week boundaries.
- Informational only.
Frequently asked questions
When does each trimester start and end?
First trimester: weeks 1-13. Second trimester: weeks 14-27. Third trimester: weeks 28 until birth (around week 40).
Do all sources agree on trimester boundaries?
The exact week boundaries vary slightly between sources (e.g. some end the first trimester at week 12 rather than 13), but the differences are minor and don't change clinical care.
What changes between trimesters?
Each trimester is associated with different developmental milestones for the baby and different common symptoms and screening tests for the pregnant person — ask your healthcare provider what to expect at each stage.
Related calculators
Sources
- Methods for Estimating the Due Date — American College of Obstetricians and Gynecologists (ACOG). Effective 01-05-2017, reviewed 13-09-2026.
This calculator provides a general estimate only and does not constitute personalized medical advice. Always confirm your pregnancy progress with a qualified healthcare provider.
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