About this calculator
The Conception Date Calculator estimates a likely conception date and conception window from either an estimated due date or the first day of the last menstrual period. It is designed for general pregnancy dating context, especially when comparing due-date, LMP, ovulation, and fertile-window assumptions.
Conception date method
Pregnancy dating commonly counts 280 days from the first day of the last menstrual period. Conception is often estimated about 266 days before the due date, or about 14 days after LMP in a typical 28-day cycle.
- from due date: conception date = estimated due date - 266 days
- from due date: LMP = estimated due date - 280 days
- from LMP: ovulation estimate = LMP + 14 days
- estimated conception window = LMP + 11 days to LMP + 21 days
How to use the conception date calculator
- Choose whether you want to calculate from a due date or from last menstrual period.
- Enter the estimated due date if you have one from a scan or pregnancy calculator.
- Or enter the first day of the last menstrual period.
- Review the approximate conception date.
- Review the wider likely conception window.
- Compare the result with ovulation, cycle length, IVF, or ultrasound information where available.
Worked examples
From due date
Input: Estimated due date 1 December 2026
Calculation: 1 December 2026 - 266 days
Result: Approximate conception date around 10 March 2026.
From last period
Input: LMP 1 March 2026
Calculation: LMP + 14 days, with a wider LMP + 11 to +21 day window
Result: Estimated ovulation around 15 March with a likely window from 12 to 22 March.
Why conception dating is only approximate
Ovulation does not happen on the same day for everyone. A 28-day cycle with ovulation around day 14 is a useful teaching model, but cycle length, luteal phase length, stress, illness, breastfeeding, hormonal contraception changes, and irregular periods can all shift timing.
Sperm can survive for several days in the reproductive tract, so intercourse date and conception date are not always the same date.
Due date, LMP, and ultrasound dating
- LMP dating
- Counts pregnancy from the first day of the last menstrual period, which is usually about two weeks before conception.
- Due date dating
- Works backwards from an estimated due date, often using 266 days from conception to due date.
- Ultrasound dating
- Early ultrasound dating may be used by clinicians when cycle dates are uncertain or do not match measurements.
Situations where dates can differ
- Irregular cycles can move ovulation earlier or later.
- IVF and assisted conception use different dating rules based on transfer date and embryo age.
- Early ultrasound may revise an estimated due date.
- Bleeding in early pregnancy is not always a true period.
Pregnancy dating disclaimer
This guide is general information only and is not medical advice. Conception dates are estimates and should not replace advice from a midwife, GP, fertility clinic, or maternity unit.
Frequently asked questions
Can the calculator tell the exact conception date?
No. It estimates a likely date and window from standard pregnancy dating assumptions. Exact conception timing is usually not knowable from dates alone.
Why is pregnancy counted from LMP rather than conception?
LMP is often easier to identify than ovulation or fertilisation, so pregnancy dating traditionally counts from the first day of the last period.
Does a due date prove when conception happened?
No. A due date can suggest a likely conception window, but cycle variation and scan dating can change the estimate.
What if I know my ovulation date?
A known ovulation date may be more specific than a generic day-14 estimate, but clinical dating should still be discussed with your care team.
Can IVF pregnancies use this calculator?
IVF dating normally uses egg collection, fertilisation, or embryo transfer details. Use an IVF-specific due date calculator or clinic advice for that situation.
Related calculators
- due-date-calculator
- pregnancy-conception-calculator
- ovulation-calculator