Pregnancy Conception Calculator
Estimate a conception date from a due date or from the first day of the last menstrual period. The model uses conception = LMP + 14 days, which is the same as due date − 266 days.
Pregnancy Conception Calculator tool
About the Pregnancy Conception Calculator
Fertilization can occur a day or two after ovulation, and sperm can survive several days. The date shown is a midpoint for a 28-day cycle, not a paternity test or a legal finding.
Pair this with the pregnancy and due date calculators if you also want gestational age or an ultrasound-based date. A several-day fertile window is more realistic than a single highlighted day, especially if cycle length is not 28 days or ovulation was not confirmed. Dates are UTC calendar days, so a timezone does not shift the printed day. This is not a legal or genetic finding.
How the Pregnancy Conception Calculator works
Both modes recover the same LMP-based timeline and then report LMP + 14 as conception.
From LMP: conception = LMP + 14 days
From due date: LMP = due − 280; conception = due − 266
Example
A due date of 8 October 2026 implies LMP 1 January 2026 and conception 15 January 2026.
How to Use the Pregnancy Conception Calculator
- Choose whether you know the due date or the LMP.
- Enter that date.
- Set the as-of date if you also want gestational age.
- Read the estimated conception date.
- Treat a several-day window as more realistic than a single day.
Pregnancy Conception Calculator Examples
From due date
Due 8 October 2026 → conception 15 January 2026.
From LMP
LMP 1 January 2026 → conception 15 January 2026.
IVF dating
If you know the retrieval or transfer date, that clinical date is better than this 14-day offset. This tool does not model embryo age at transfer.
How to Estimate Conception
If you know LMP, add 14 days. If you know the due date, subtract 266 days. Both come from the same 40-week obstetric clock.
Same timeline, two entries
These rows are the January 2026 example.
| You enter | Conception | Due date |
|---|---|---|
| LMP 2026-01-01 | 2026-01-15 | 2026-10-08 |
| Due 2026-10-08 | 2026-01-15 | 2026-10-08 |
Fertile window
A common teaching window is about five days before ovulation through the day of ovulation. The single date this tool prints sits in the middle of that story, not at its edges. If a clinic dated the pregnancy from ultrasound or from an IVF transfer, prefer that clinical date over this 14-day offset. The page is not a paternity tool and does not ask for cycle length, so a long follicular phase will push real ovulation later than LMP + 14. Use the pregnancy calculator when you also want trimester labels, and the due-date calculator when you have an ultrasound age to enter.
Frequently Asked Questions
Is this exact?
No. It assumes ovulation on day 14 of a 28-day cycle.
Can I use it for paternity?
No. It is not a legal or genetic test.
Why 266 days?
280 days from LMP minus the conventional 14 days to ovulation.
Does cycle length change this?
A longer follicular phase pushes ovulation later. This page does not ask for cycle length.
Medical advice?
No.