Pregnancy Conception Window
Estimate a conception window from a due date or last menstrual period, using the standard 280-day pregnancy model. Educational timing, not a fertility diagnosis.
Results
Estimated conception
Conception window
- Implied LMP
- March 10, 2026
- Due date (Naegele ~280d)
- December 15, 2026
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Educational estimates · not financial advice · free, no signup
Formula / how it works
Due date ≈ LMP + 280 days. LMP ≈ due date − 280 days. Conception is estimated around ovulation, about 14 days after the LMP in a 28-day cycle, adjusted by cycle length.
Detailed guide
What it is, how to use it, how to read the numbers, common mistakes, a worked example, formulas, and FAQs.
What this calculator is
Pregnancy dating is traditionally counted from the first day of the last menstrual period, not from conception. By convention, a due date is 280 days, or 40 weeks, after that date. Conception usually happens about two weeks after the LMP, around ovulation, so the conception date is roughly 266 days before the due date in a regular 28-day cycle. This calculator works backward from a due date, or forward from an LMP, and shows an estimated conception date and a window around it, the implied LMP and the Naegele due date. Real ovulation varies from cycle to cycle, sperm can survive for several days, and ultrasound dating may differ, so treat the dates as estimates. Confirm dating with a clinician.
When to use / who it helps
- You know a due date and want the approximate conception date.
- You know your LMP and cycle length and want a due date.
- You are curious how clinicians date a pregnancy.
- You are reading a pregnancy timeline and want to translate it.
- You want a window rather than a single day.
How to use
Steps
- Choose whether the known date is a due date or an LMP.
- Enter the date.
- Enter your usual cycle length in days.
- Read the estimated conception date and window.
- Read the implied LMP and due date.
- Compare with your clinician's dating from ultrasound.
How to interpret results
- The conception date is an estimate centred on ovulation.
- The window reflects uncertainty of a few days either side.
- Cycle lengths longer or shorter than 28 days shift ovulation.
- Ultrasound dating can differ from LMP dating by days.
- Only about 4% of babies arrive on the due date.
Common mistakes & gotchas
- Assuming conception happened exactly on the estimated day.
- Using LMP when cycles are very irregular.
- Entering the conception date instead of the LMP.
- Ignoring cycle length when it differs from 28 days.
- Using the tool as a fertility test.
Worked example
Worked example — due date 15 December 2026, 28-day cycle
- Implied LMP: 15 December 2026 − 280 days = 10 March 2026.
- Estimated conception: LMP + 14 days = 24 March 2026.
- The tool shows a window of 21 to 26 March 2026 around that date.
- Check: 10 March + 280 days is 15 December 2026.
Result: Estimated conception around 24 March 2026, with an implied LMP of 10 March 2026.
To load the form's own demo values, tap Try example above the Calculate button.
How to calculate / formula
Due date ≈ LMP + 280 days. LMP ≈ due date − 280 days. Conception is estimated around ovulation, about 14 days after the LMP in a 28-day cycle, adjusted by cycle length.
Frequently asked questions
How is the due date calculated?
Add 280 days to the first day of the last menstrual period (Naegele's rule).
When does conception happen?
Around ovulation, typically about 14 days after the LMP in a 28-day cycle.
Why is there a window?
Ovulation timing varies and sperm can survive for several days.
Is the estimate exact?
No. It is an educational model; an ultrasound can refine dating.
What if my cycle is not 28 days?
Enter your usual cycle length; the estimate shifts accordingly.
Is this medical advice?
No. Confirm dates with your healthcare provider.
For education and personal goal-setting only — not medical advice, diagnosis, or treatment. Talk to a qualified clinician about your health. Full disclaimer