Conception Date | Cycle, Due Date & Ovulation

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.

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

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.

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.

How the Math Works

All three input methods converge on the same core arithmetic. From a confirmed due date, conception is estimated as due date minus 266 days (pregnancy from fertilization averages 38 weeks). From last menstrual period (LMP), the calculator first estimates ovulation day as LMP + (cycle length − 14), since the luteal phase averages 14 days regardless of total cycle length. Conception is assumed to occur on or near ovulation day. From an ultrasound, the scan date and gestational age at scan are used to derive a due date, then the 266-day subtraction is applied. The probability window extends 3 days before to 1 day after the estimated conception date, accounting for sperm viability of up to 5 days and the egg's short lifespan of roughly 24 hours. Implantation is forecast at 6–12 days after estimated conception, when the embryo attaches to the uterine wall.

How to Use This Calculator

Choose the input method that matches the most accurate information you have: Due Date if your provider has confirmed an estimated due date, Last Period (LMP) if you know the first day of your most recent period, or Ultrasound if you have a scan date with gestational age. For the LMP method, also enter your average cycle length so the calculator can adjust the ovulation estimate for cycles longer or shorter than 28 days. The results show your estimated conception date, a probability window (the range of days when intercourse most likely led to conception), your current gestational age, and an implantation forecast. When ultrasound data is available, use it, early first-trimester ultrasound is generally more precise than LMP alone for establishing gestational age and the derived conception date.

Biology vs Medicine: Conception vs Gestational Age

Conception (Fertilization) vs Gestational Age

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.

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.

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.

Sources & citations

References used for the calculation method and definitions. Links open in a new tab when available.

[1]

ACOG Committee Opinion No. 700: Methods for Estimating the Due Date

ACOG clinical guidance on gestational age calculation and the relationship between LMP, ultrasound measurements, and estimated conception date.

[2]

Wilcox AJ, Weinberg CR, Baird DD. Timing of Sexual Intercourse in Relation to Ovulation

New England Journal of Medicine study (1995) establishing conception probability relative to ovulation day, used clinically to estimate likely conception dates.

Pregnancy Conception Calculator: When Did I Conceive?

Method

Timeline

Probability window (likely sex window)

Implantation forecast

Current gestational age

Flow

When did I conceive?