Pregnancy Calculator
Your pregnancy timeline from your last menstrual period.
Result
Fill in the inputs to see your result.
Inputs
Formula
Naegele's rule: due date = LMP + 280 days, adjusted for cycle length.
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Calcxo gives you the answer plus the reasoning behind it.
Quick answer
Pregnancy Calculator: in one paragraph
A pregnancy calculator estimates your due date and how many weeks pregnant you are, usually by counting 280 days (40 weeks) from the first day of your last menstrual period (LMP).
What this calculator does
Enter the first day of your last period (or a known conception or ultrasound date) and the calculator returns your estimated due date, current gestational age in weeks and days, and which trimester you're in.
Why people use it
- Get an at-a-glance estimated due date (EDD) without doing the math by hand.
- Track gestational age in weeks for appointments and milestones.
- Plan around trimesters, screenings, and parental leave.
- Cross-check what a midwife or doctor has told you.
A worked due-date calculation
A concrete walkthrough you can follow line by line.
Your last period started on 1 February 2026 with a typical 28-day cycle.
1. Add 280 days to LMP
1 Feb 2026 + 280 days = 8 Nov 2026
2. Or use Naegele's rule
LMP + 1 year − 3 months + 7 days = 8 Nov 2026
3. Today's gestational age
Days since LMP ÷ 7 = current weeks and days pregnant
Result
Estimated due date: 8 November 2026 — give or take about two weeks either side.
When to use this calculator
- Right after a positive test, to get an early due-date estimate.
- When swapping between LMP-based and ultrasound-based dates.
- For planning purposes only — clinical care should rely on your provider's dating.
Common mistakes
- Using the date you got a positive test instead of LMP — they can differ by weeks.
- Assuming a 28-day cycle when yours is shorter or longer.
- Treating the due date as a fixed deadline — only ~5% of babies arrive on it.
- Sticking with an LMP-based date after an early ultrasound has revised it.
Quick questions
How accurate is the due date?+
It's an estimate. Most births occur within a 5-week window around the EDD. Early ultrasounds (before 13 weeks) are the most accurate way to date a pregnancy.
What if I don't know my LMP?+
You can use a known conception date (add 266 days) or ask for an early dating ultrasound — that's the gold standard when LMP is uncertain.
Learn more
Deeper reads from the Calcxo learn library.
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Revision history2 changes · last on Feb 18, 2026⌄
- ReviewedReviewed Naegele's rule implementation and edge cases.
- ReleasedInitial release.