Conception is when sperm fertilizes the egg — usually near ovulation, about 11–21 days after the first day of your last period on a typical cycle, or about 266 days before your due date. Default example: EDD 2026-07-08 → conception ≈ 2025-10-15. Possible intercourse that could have led to pregnancy: Oct 10, 2025 through Oct 15, 2025 (sperm may survive up to ~5 days). Exact fertilization moment is rarely known.

How is conception date calculated from due date?

Subtract 266 days from the estimated due date. Pregnancy is dated as 280 days from LMP, but conception is typically ~14 days after LMP, so 280 − 14 = 266. Example: EDD 2026-07-08 − 266 days = 2025-10-15. This matches LMP Oct 1, 2025 with a 28-day cycle (conception 2025-10-15, EDD 2026-07-08).

What is the difference between conception and fertilization?

Fertilization is sperm meeting egg in the fallopian tube. Conception in everyday use often means the full process including implantation; clinically, pregnancy is usually recognized after implantation. This calculator estimates the fertilization window (~2025-10-15 in the default case), not the day a home test turned positive (often ~10–14 days later).

How accurate are conception date estimates?

Approximate only. Ovulation can shift cycle to cycle; implantation adds variability. First-trimester ultrasound back-dating is usually the most reliable method. Example: scan Feb 1, 2026 at 12+0 → conception ≈ 2025-11-23 (EDD 2026-08-16). A mid-pregnancy scan at 22+6 on Mar 15, 2026 → conception ≈ 2025-10-20 — less precise for conception than early scans.

What is the possible range of intercourse dates?

Sperm can live about 3–5 days in the reproductive tract, so intercourse up to five days before ovulation/conception may still result in pregnancy. Default window: 2025-10-10 to 2025-10-15 (6 calendar days including conception day). The egg is viable roughly 12–24 hours after ovulation, so the fertile window is narrower than the sperm survival window.

Why is conception date different from gestational age?

Gestational age counts from the first day of LMP — about two weeks before fertilization. If gestational age is 10 weeks today, fetal age from conception is about 8 weeks. Our pregnancy calculator reports gestational weeks; this tool estimates the underlying conception date (default 2025-10-15 for EDD 2026-07-08).

How does cycle length affect conception date?

Ovulation is modeled about 14 days before the next period, so conception ≈ LMP + (cycle length − 14). Same LMP Oct 1, 2025: 28-day cycle → conception 2025-10-15; 35-day cycle → conception 2025-10-22 (7 days later) and EDD 2026-07-15 vs 2026-07-08. Longer cycles usually mean later ovulation and later conception.

Can I determine conception from an ultrasound?

Yes — enter scan date and gestational age (weeks + days). We compute EDD = scan + (280 − gestational days), then conception ≈ EDD − 266. Early ultrasound 12+0 on Feb 1, 2026 → 2025-11-23, intercourse ~Nov 18, 2025–Nov 23, 2025. Dating scans before ~13 weeks are preferred over later growth scans for conception estimates.

Is conception date the same as the date of intercourse?

Not always. Conception in this tool is the estimated fertilization day; intercourse could have been up to ~5 days earlier. Default: conception 2025-10-15, but sex as early as 2025-10-10 might still fit. Intercourse after conception day does not cause that cycle’s pregnancy.

What if I have irregular periods?

LMP-based conception math assumes a predictable luteal phase. Irregular cycles make LMP-only estimates unreliable — use an early ultrasound or a clinician-set due date. Entering EDD 2026-07-08 avoids guessing cycle length; entering LMP still helps if you know a typical cycle length (try 28 vs 35 day examples above).

How does this relate to the ovulation calculator?

The ovulation calculator predicts fertile days forward from LMP. This tool works backward from pregnancy dating (due date, LMP, or scan) to estimate when conception likely occurred. If you are trying to conceive, use ovulation timing; if you are already pregnant and asking “when did I conceive?”, use this calculator.

Is this medical or legal advice?

No. Educational estimates only — not for paternity disputes, legal proceedings, or clinical decisions. Your prenatal provider sets official gestational age and may adjust dating after ultrasound.

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When did I conceive?

Pregnancy Conception Calculator — Conception & Intercourse Dates

Free conception date calculator: work backward from due date, last period, or ultrasound. Default EDD July 8, 2026 → conception ≈ Oct 15, 2025 · intercourse window Oct 10–15.

Last updated: May 25, 2026 · Educational only — not medical or legal advice

Conception ≈ 2025-10-15 · EDD 2026-07-08

Intercourse window Oct 10, 2025 – Oct 15, 2025

Sperm viability up to ~5 days modeled

Pregnancy calculator Ovulation

Pregnancy Conception Calculator

Estimate conception date and possible intercourse dates

Calculate based on

Your due date

Conception ≈ 2025-10-15 · EDD 2026-07-08 · intercourse window Oct 10, 2025 – Oct 15, 2025 (up to 5 days before).

Estimated conception

Wednesday, October 15, 2025

Possible intercourse dates

Oct 10, 2025toOct 15, 2025

Up to 5 days before conception (typical viability ~3–5 days)

Conservative 3-day window from 2025-10-12

Estimated due date

Wednesday, July 8, 2026

Estimates only — not for legal or medical diagnosis. See our pregnancy calculator for full EDD and trimester tracking.

Default snapshot (EDD July 8, 2026)

Conception

2025-10-15

Intercourse window

2025-10-10 → 2025-10-15

Due date

2026-07-08

Formulas used

Due date: Conception ≈ EDD − 266 days (280-day pregnancy minus ~14 days LMP-to-ovulation)

Last period (LMP): EDD = LMP + 280 + (cycle − 28); conception ≈ LMP + (cycle − 14) assuming 14-day luteal phase

Ultrasound: EDD = scan date + (280 − gestational days at scan); conception ≈ EDD − 266

Intercourse range: conception date − 5 days through conception date

Fertilization vs conception (quick guide)

Fertilization

Sperm meets egg in the fallopian tube — the biological moment this calculator estimates (≈ 2025-10-15 in the default example).

Implantation & pregnancy

The embryo attaches to the uterus several days after fertilization. Home pregnancy tests often turn positive about 10–14 days after conception, not on conception day itself.

Worked scenarios

Default — known due date

EDD July 8, 2026 → conception ≈ Oct 15, 2025

Conception 2025-10-15

EDD 2026-07-08

Intercourse Oct 10, 2025 – Oct 15, 2025

LMP Oct 1, 2025 · 28-day cycle

Ovulation ~day 14 → conception Oct 15

Conception 2025-10-15

EDD 2026-07-08

Intercourse Oct 10, 2025 – Oct 15, 2025

LMP · 35-day cycle

Later ovulation → conception Oct 22

Conception 2025-10-22

EDD 2026-07-15

Intercourse Oct 17, 2025 – Oct 22, 2025

Early ultrasound 12+0

Scan Feb 1, 2026 at 12 weeks

Conception 2025-11-23

EDD 2026-08-16

Intercourse Nov 18, 2025 – Nov 23, 2025

Mid-pregnancy ultrasound 22+6

Later scans are less precise for conception dating

Conception 2025-10-20

EDD 2026-07-13

Intercourse Oct 15, 2025 – Oct 20, 2025

Limitations

Conception date is an estimate — fertilization timing varies with ovulation, sperm survival, and implantation
Intercourse window assumes sperm may survive up to ~5 days; egg viability ~12–24 hours after ovulation
Not for paternity, legal, or medical diagnosis — consult your clinician for official dating
First-trimester ultrasound dating is more reliable than mid-pregnancy scans for back-calculating conception
IVF conceptions use transfer-date rules — use our pregnancy calculator IVF mode instead

References

Not medical or legal advice. Conception and intercourse dates are estimates. For official pregnancy dating, prenatal care, or legal questions, consult qualified professionals.

Frequently Asked Questions

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When did I conceive?

Pregnancy Conception Calculator — Conception & Intercourse Dates

Last updated: May 25, 2026 · Educational only — not medical or legal advice

Conception ≈ 2025-10-15 · EDD 2026-07-08

Intercourse window Oct 10, 2025 – Oct 15, 2025

Sperm viability up to ~5 days modeled

Pregnancy calculator Ovulation

Pregnancy Conception Calculator

Estimate conception date and possible intercourse dates

Calculate based on

Your due date

Conception ≈ 2025-10-15 · EDD 2026-07-08 · intercourse window Oct 10, 2025 – Oct 15, 2025 (up to 5 days before).

Estimated conception

Wednesday, October 15, 2025

Possible intercourse dates

Oct 10, 2025toOct 15, 2025

Up to 5 days before conception (typical viability ~3–5 days)

Conservative 3-day window from 2025-10-12

Estimated due date

Wednesday, July 8, 2026

Estimates only — not for legal or medical diagnosis. See our pregnancy calculator for full EDD and trimester tracking.

Default snapshot (EDD July 8, 2026)

Conception

2025-10-15

Intercourse window

2025-10-10 → 2025-10-15

Due date

2026-07-08

Formulas used

Due date: Conception ≈ EDD − 266 days (280-day pregnancy minus ~14 days LMP-to-ovulation)

Last period (LMP): EDD = LMP + 280 + (cycle − 28); conception ≈ LMP + (cycle − 14) assuming 14-day luteal phase

Ultrasound: EDD = scan date + (280 − gestational days at scan); conception ≈ EDD − 266

Intercourse range: conception date − 5 days through conception date

Fertilization vs conception (quick guide)

Fertilization

Sperm meets egg in the fallopian tube — the biological moment this calculator estimates (≈ 2025-10-15 in the default example).

Implantation & pregnancy

The embryo attaches to the uterus several days after fertilization. Home pregnancy tests often turn positive about 10–14 days after conception, not on conception day itself.

Worked scenarios

Default — known due date

EDD July 8, 2026 → conception ≈ Oct 15, 2025

Conception 2025-10-15

EDD 2026-07-08

Intercourse Oct 10, 2025 – Oct 15, 2025

LMP Oct 1, 2025 · 28-day cycle

Ovulation ~day 14 → conception Oct 15

Conception 2025-10-15

EDD 2026-07-08

Intercourse Oct 10, 2025 – Oct 15, 2025

LMP · 35-day cycle

Later ovulation → conception Oct 22

Conception 2025-10-22

EDD 2026-07-15

Intercourse Oct 17, 2025 – Oct 22, 2025

Early ultrasound 12+0

Scan Feb 1, 2026 at 12 weeks

Conception 2025-11-23

EDD 2026-08-16

Intercourse Nov 18, 2025 – Nov 23, 2025

Mid-pregnancy ultrasound 22+6

Later scans are less precise for conception dating

Conception 2025-10-20

EDD 2026-07-13

Intercourse Oct 15, 2025 – Oct 20, 2025

Limitations

Conception date is an estimate — fertilization timing varies with ovulation, sperm survival, and implantation
Intercourse window assumes sperm may survive up to ~5 days; egg viability ~12–24 hours after ovulation
Not for paternity, legal, or medical diagnosis — consult your clinician for official dating
First-trimester ultrasound dating is more reliable than mid-pregnancy scans for back-calculating conception
IVF conceptions use transfer-date rules — use our pregnancy calculator IVF mode instead

References

Not medical or legal advice. Conception and intercourse dates are estimates. For official pregnancy dating, prenatal care, or legal questions, consult qualified professionals.