Conception Calculator
Calculate conception dates and plan your fertility timing
Conception calculator in 8 quick sections
1) Core idea
Either work backward from a due date/pregnancy week to estimate conception, or work forward from last period to plan fertile days.
2) Flow you can trust
- Reverse mode → Conception ≈ Due date − 266 days (≈38 weeks).
- Weeks mode → LMP = today − (weeks × 7), conception ≈ LMP + 14, due ≈ LMP + 280.
- Plan mode → Ovulation ≈ last period + (cycle − 14); fertile window = ovulation −5 to +1 day.
3) Sanity checks
- Sperm can survive up to ~5 days; implantation adds ~6–12 days variability.
- Ultrasound dating early in pregnancy is usually most accurate.
4) Shortcuts
- No dates handy? Use weeks pregnant to back‑calculate LMP and conception.
- Planning around a desired due date? Target conception ≈ 266 days earlier.
5) Pitfalls
- Assuming ovulation day is exact—treat as an estimate.
- Forgetting luteal length can vary among individuals.
6) Micro examples
- Due date Oct 1 → conception ≈ Jan 9 (−266 days).
- 8 weeks pregnant today → LMP ≈ 56 days ago; conception ≈ 14 days after that.
7) Mini‑FAQ
- Which is more accurate? Early ultrasound > LMP guesses.
- Why a window? Intercourse day, sperm lifespan, and implantation timing vary.
- Trying to conceive? Time intercourse every other day in the fertile window.
8) Action tip
Use this as a planning guide, then confirm with ovulation tests or medical dating. Treat all dates as estimates.