Pregnancy Trimester Calculator
Trimester dates, current week and countdown from your last period or due date.
Enter your values
Enter the first day of last period you are working with.
Enter the cycle length in days.
Enter the date to check you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Current stage
- Second trimester, week 13 day 1
- Estimated due date
- 2026-12-06
- First trimester
- 2026-03-01 to 2026-05-30
- Second trimester
- 2026-05-31 to 2026-09-05
- Third trimester
- 2026-09-06 to 2026-12-05
- Days remaining
- 188 days (26.9 weeks)
- Viability milestone (24 weeks)
- 2026-08-16
Working
- 1due = LMP + 280 days = 2026-12-06
Quick answer
Trimesters split a 40 week pregnancy at weeks 13 and 27, counted from the first day of the last menstrual period rather than conception.
due date = LMP + 280 days
At a glance
| What it does | Trimester dates, current week and countdown from your last period or due date. |
|---|---|
| Category | Health |
| Inputs needed | First day of last period, Cycle length, Date to check |
| Main output | Current stage |
| Formula | due date = LMP + 280 days |
| Cost | Free — no sign-up, no download |
How to use the Pregnancy Trimester Calculator
- 1Enter the first day of your last menstrual period in YYYY-MM-DD format.
- 2Adjust cycle length if yours is not 28 days — the due date shifts by the difference.
- 3Change the date to check to see where you were or will be at any point.
Inputs explained
- First day of last period
- Enter the first day of last period you are working with.
- Cycle length(days)
- Enter the cycle length in days.
- Date to check
- Enter the date to check you are working with.
Worked example
Using the values the calculator loads with:
Inputs
- First day of last period2026-03-01
- Cycle length28 days
- Date to check2026-06-01
Results
- Current stageSecond trimester, week 13 day 1
- Estimated due date2026-12-06
- First trimester2026-03-01 to 2026-05-30
- Second trimester2026-05-31 to 2026-09-05
- Third trimester2026-09-06 to 2026-12-05
- Days remaining188 days (26.9 weeks)
- Viability milestone (24 weeks)2026-08-16
- due = LMP + 280 days = 2026-12-06
Frequently asked questions
Why count from the last period and not conception?
Ovulation date is rarely known, but the last period usually is. Clinical dating has used LMP plus 280 days for over a century.
Does an ultrasound date override this?
Yes. A first-trimester scan is more accurate than LMP dating, so use the scan date your clinician gives you if they differ.
What do I need to enter into the Pregnancy Trimester Calculator?
Just 3 values: first day of last period, cycle length, date to check. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Pregnancy Trimester Calculator work out the answer?
Trimesters split a 40 week pregnancy at weeks 13 and 27, counted from the first day of the last menstrual period rather than conception. It applies the formula due date = LMP + 280 days and shows the working so you can check each step by hand.
Is the Pregnancy Trimester Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.