Conception Date Calculator: When Did I Conceive?

Fertilization Timeline

Estimated conception (fertilization)

Wednesday, December 17, 2025

The window when egg and sperm most likely merged. Not the same as the sex date—sperm can survive several days.

Probability window (likely sex window)

3 days before to 1 day after estimated conception—sperm can survive up to 5 days inside the body.

Dec 14, 2025 – Dec 18, 2025

Implantation forecast

~6–12 days after conception, when the embryo attaches to the uterine wall.

Dec 23, 2025 – Dec 29, 2025

Current gestational age

12w 0d

Measured from the first day of your last period (clinical standard), not from conception.

Timeline

LMP→Dec 3, 2025→Conception window→Dec 17, 2025→Implantation ~6–12 days later→Today (12w 0d)

Pregnancy Conception Calculator: Fertilization Timeline

Why does "when did I conceive?" feel harder to answer than "when am I due?" Because conception is a biological event that can't be seen—and the calendar that medicine uses starts two weeks earlier. These insights explain why a single conception day is the wrong target, why biology gives you a window instead, and when to trust a scan over LMP.

Conception Intelligence

Why "Weeks Pregnant" Starts Before Conception

•The clinical calendar doesn't start at fertilization—it starts at your last period.

•That's why "4 weeks pregnant" means you've only just conceived. The calculator gives you both: the biological start (estimated conception) and the clinical number (gestational age from LMP). Once you see both, the mismatch between "when did I conceive?" and "how many weeks am I?" stops feeling confusing.

Why You Get a Window, Not a Day

•Biology doesn't hand you a timestamp.

•Sperm can wait for the egg; the egg doesn't wait long. So the act that led to conception might have been several days before fertilization. The tool shows a probability window for that reason—so you stop hunting for one exact day and instead see the range that actually fits biology.

When to Trust the Scan Over LMP

•If your provider gave you a due date from an early ultrasound, that date usually beats LMP alone.

•LMP assumes a "typical" cycle and ovulation; real cycles vary. The calculator lets you switch to due date or ultrasound when you have that data—so your conception window reflects the best information you have, not just the calendar guess.

Pregnancy Conception Calculator: When Did I Conceive? | 2026

Pregnancy conception calculator and conception date calculator: how to find my conception date from due date, last period, or ultrasound. Conception window vs due date, how many days after period did I conceive, ultrasound vs period date for conception. Client-side only—no data sent.

What This Pregnancy Conception Calculator Does

Calculator Purpose & Outputs

What You'll Get:

Estimated Conception: Typically 11–21 days after LMP (or derived from due date − 266 days, or from ultrasound). The window when egg and sperm most likely merged.
Probability Window (Sex Window): 3 days before to 1 day after estimated conception—accounts for sperm viability (up to 5 days). Conception date is not the same as the sex date.
Current Gestational Age: Calculated from the first day of your last period (clinical standard).
Implantation Forecast: ~6–12 days after conception, when the embryo attaches to the uterine wall.
Privacy: No sensitive pregnancy data sent to any server—client-side only.
Calculation Methods:

Due Date: Conception = Due Date − 266 days (pregnancy from fertilization is ~38 weeks).
Last Period (LMP): Conception ≈ LMP + (cycle length − 14) for typical ovulation (~14 days before next period). Adjustable by cycle length.
Ultrasound: Enter scan date and gestational age at scan; EDD is derived, then conception = EDD − 266 days. Ultrasound vs period date for conception: early ultrasound is often more accurate than LMP alone.
Ideal Users:

Anyone asking "When did I conceive?" or "How to find my conception date?": Use due date, LMP, or ultrasound to get your estimated conception and probability window.
Conception window vs due date: The calculator shows how your conception window relates to your due date (conception + 266 days ≈ EDD).
"How many days after period did I conceive?": Typically 11–21 days after LMP; the tool uses your cycle length to refine the estimate.
Ultrasound vs period date for conception: Choose ultrasound when you have scan data for higher accuracy.
Accuracy & Limitations:

Conception cannot be pinpointed to a single day—sperm viability and ovulation timing create a probability window. Early first-trimester ultrasound is the most accurate clinical method for gestational age; this tool is for estimation and education only, not a substitute for prenatal care.

This pregnancy conception calculator (when did I conceive? tool) reverse-engineers your likely conception (fertilization) date from your due date, first day of last period (LMP), or ultrasound. It provides a fertilization timeline—estimated conception, probability window (sex window), implantation forecast, and current gestational age—rather than a single static day.

Biology vs Medicine: Conception vs Gestational Age

Conception (Fertilization) vs Gestational Age

Conception typically 11–21 days after LMP:

Conception occurs around ovulation, which is usually 14 days before your next period. For a 28-day cycle that's ~14 days after LMP; for other cycles use LMP + (cycle length − 14). How many days after period did I conceive? The tool uses your cycle length to refine the estimate.
Is conception date the same as the sex date?

No. Sperm can live up to 5 days; the probability window is 3 days before to 1 day after estimated conception. Conception window vs due date: conception + 266 days ≈ EDD.
Ultrasound vs period date for conception:

Early first-trimester ultrasound is the most accurate clinical method for gestational age. When you have scan date and weeks/days at scan, use the ultrasound option in this pregnancy conception calculator for a more precise conception window than LMP alone.

Medical professionals count "weeks pregnant" from the first day of your last period—about 2 weeks before conception. Conception is the biological start (egg and sperm merge); gestational age is the clinical standard. This pregnancy conception calculator gives you both so you can answer "when did I conceive?" and "how many weeks am I?" in one place.

Pregnancy Conception Calculator FAQ

? Can I know the exact day I conceived?

Not with certainty. Conception (fertilization) is estimated from your due date (due date − 266 days), last period (LMP + ~14 days for a 28-day cycle), or ultrasound. Sperm can survive up to 5 days, so the probability window is typically 3 days before to 1 day after the estimated conception date. This pregnancy conception calculator gives you that window—not a single definitive day.

? How does ultrasound help find the conception date?

An early first-trimester ultrasound measures gestational age (from LMP). From that, your due date is derived; conception = due date − 266 days. Ultrasound is the most accurate clinical method for gestational age—often more accurate than LMP alone—so it improves the precision of your estimated conception window. Use the ultrasound option in this pregnancy conception calculator when you have scan date and weeks/days at scan.

? Is conception date the same as the sex date?

No. Conception is when the egg and sperm merge (fertilization). Intercourse may have occurred 1–5 days earlier—sperm can survive up to 5 days inside the body. So the "sex window" is typically 3 days before to 1 day after the estimated conception date. This pregnancy conception calculator shows that probability window so you understand the range of possible dates.

? How to find my conception date?

Use your due date, last period, or ultrasound. Due date: conception ≈ due date − 266 days. Last period: conception ≈ first day of LMP + (cycle length − 14) for typical ovulation. Ultrasound: enter scan date and gestational age at scan; the calculator derives your due date then subtracts 266 days. This pregnancy conception calculator supports all three methods and shows a probability window (3 days before to 1 day after) and implantation forecast (6–12 days after conception).

? How many days after my period did I conceive?

Conception typically occurs 11–21 days after the start of your last period—around ovulation, which is usually 14 days before your next period. For a 28-day cycle, that's about 14 days after LMP. For longer or shorter cycles, ovulation shifts: conception ≈ LMP + (cycle length − 14). This pregnancy conception calculator uses your cycle length to estimate when you conceived and shows the probability window (sperm viability) and implantation range (6–12 days after conception).

? Ultrasound vs period date for conception—which is more accurate?

Early first-trimester ultrasound is generally more accurate than LMP alone for dating. LMP assumes regular cycles and day-14 ovulation; irregular cycles or late ovulation can shift the estimate. Ultrasound measures fetal size and gives a gestational age that is then used to derive due date and conception. When you have both, use the ultrasound option in this pregnancy conception calculator for a more precise conception window.

Calculation Method