Calculation details
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What does this calculator estimate?
A pregnancy calculator estimates your due date and current stage from your last menstrual period. Enter the LMP to see the due date, weeks along, and trimester.
- Pregnancy ≈ 40 weeks from LMP
- Due date = LMP + 280 days
- Third trimester starts at 28 weeks
How pregnancy dating works
Pregnancy is dated from the first day of the last menstrual period, even though conception happens about two weeks later. Forty weeks (280 days) from the LMP is the standard due date; the current week and trimester follow from today's date.
Limitations to watch for
The estimate assumes a regular 28-day cycle and day-14 ovulation. Only about 5% of babies arrive on the estimated due date. Early ultrasound dating is more accurate and may adjust the date. Multiples often deliver earlier.
How to use it in practice
Use the estimate to plan appointments, maternity leave, and the third-trimester window. Confirm the dating with your provider's ultrasound. Expect labor anywhere from 37 to 42 weeks — the due date is the midpoint, not a deadline.
['Enter the first day of your last menstrual period.', 'The tool computes the due date, weeks along, and trimester.', "Confirm with your provider's ultrasound dating."]
How this calculator works
Formula
Due date = LMP + 280 days (40 weeks). Key milestones: conception ≈ LMP + 14 days; the estimate tracks weeks of pregnancy from the LMP and flags the current trimester (first: 0–13 weeks, second: 14–27, third: 28+).
Worked example
With an LMP of February 1, 2026: due date ≈ November 8, 2026 — 40 weeks later. Conception was around mid-February.
Assumptions to verify
- A regular 28-day cycle with ovulation at day 14.
- A single pregnancy.
- The entered LMP is accurate.
Frequently asked questions
How is my due date calculated?
Add 280 days (40 weeks) to the first day of your last menstrual period. The tool also estimates conception and your current week.
What week am I in?
Count from the LMP: today minus the LMP in weeks. The tool does this and flags the trimester.
When does the third trimester start?
At 28 weeks. The first trimester is 0–13 weeks, the second 14–27, the third 28+.
Why is pregnancy 40 weeks when it feels shorter?
Dating starts at the LMP, about two weeks before conception — so the baby is roughly 38 weeks from conception at the 40-week mark.
How accurate is the due date?
Only about 5% of babies arrive exactly on it. Full-term delivery ranges from about 37 to 42 weeks.
Will my provider change the date?
Possibly — an early ultrasound in the first trimester is the most accurate dating method and may adjust the LMP-based estimate.
Do multiples have different milestones?
The dating is the same, but twins and higher-order multiples are usually delivered earlier, often planned for 37–38 weeks.
Cite this tool
BoringToolsKit. “Pregnancy Calculator.” boringtoolskit.com/pregnancy-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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