Conception window estimate
Pick the date you know: the first day of the last period or the expected due date.
General information, not medical advice. This result does not replace a doctor. If in doubt or if you have symptoms, see a health professional.
Result
Jan 24, 2026Estimated conception around
- Conception likely between Jan 22, 2026 and Jan 26, 2026.
- Intercourse that may be involved: about Jan 17, 2026 to Jan 27, 2026.
- Estimate only: it cannot give a certain day, nor identify a person.
Inputs
- Known date
- Last period
- Date
- 2026-01-10
- Cycle length
- 28
- Uncertainty margin
- 2
How it is calculated
From the period: conception ≈ ovulation = period + (cycle − 14) days.
Result
Estimated conception around
How it is calculated
- From the period: conception ≈ ovulation = period + (cycle − 14) days.
- From the due date: conception ≈ due date − 266 days (38 weeks).
- Sperm can live up to 5 days: the intercourse period starts 5 days earlier.
Example
Period on 10 January 2026, 28-day cycle ± 2: conception around 24 January, between 22 and 26 January.
Good to know
- No certain date is possible: ovulation varies from cycle to cycle.
- This tool cannot tell who the father is: only a genetic test can.
- An early ultrasound dates a pregnancy more precisely: talk to a doctor or midwife.
Sources
- ACOG, Methods for estimating the due date
- Wilcox A.J. et al. (1995), N Engl J Med
When to use it
- Estimate a possible conception period from the last period or due date.
- View a range of possible dates instead of treating the result as a certain day.
Frequently asked questions
How is the conception period estimated?
From a period date, the tool estimates ovulation from cycle length; from a due date, it counts back 266 days. Both are approximations.
Can the calculation identify the exact day of conception?
No. Ovulation timing varies, so the result is a possible date range, not a certain day.
Can this tool identify the biological father?
No. A date estimate cannot identify a person; only genetic testing can do that.