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Conception Calculator

Estimate when conception occurred based on due date, last menstrual period, or ultrasound. See fertile window and probable intercourse dates.

Key Dates

Intercourse Dates & Probability

How Conception Date Is Estimated

Conception (fertilization) typically occurs within 24 hours of ovulation, when a released egg meets sperm. Because sperm can survive up to 5 days in the female reproductive tract, the fertile window spans approximately 6 days—the 5 days before ovulation and the day of ovulation itself. This Professional Conception Calculator uses standard obstetric formulas to back‑calculate from your due date, last menstrual period (LMP), or early ultrasound to estimate the most probable date of conception and the surrounding fertile window.

Calculation Methods

  • Due Date Method: Subtracts 266 days (38 weeks) from the due date. This assumes a 280‑day gestation from LMP, with ovulation on day 14.
  • Last Menstrual Period (LMP) Method: Adds the estimated day of ovulation (cycle length minus 14) to the LMP. The luteal phase is typically fixed at 14 days.
  • Ultrasound Method: Back‑calculates from the ultrasound date and reported gestational age to determine LMP, then applies the ovulation estimate.

💡 Why a Window Rather Than an Exact Day?

Sperm can survive up to 5 days, so intercourse several days before ovulation can still result in pregnancy. The calculator shows a 6‑day window (5 days before to 1 day after the estimated conception date) to reflect this biological reality.

Gestational Age vs. Fetal Age

Medical professionals use gestational age, counted from the first day of the LMP—approximately 2 weeks before conception. Fetal (conceptional) age counts from the actual date of fertilization. An embryo at 8 weeks gestational age is actually about 6 weeks old. This calculator helps clarify both timelines.

Frequently Asked Questions

Can this calculator determine paternity?

No. It can narrow the possible conception window, but due to variability in ovulation and sperm survival, it cannot definitively establish paternity. DNA testing is the only reliable method.

How accurate is the ultrasound method?

First‑trimester ultrasounds (before 13 weeks) are the most accurate for dating pregnancy, with a margin of ±5‑7 days. Accuracy decreases later in pregnancy.

What if my cycles are irregular?

Irregular cycles make ovulation timing less predictable. The calculator uses an average cycle length; if you tracked ovulation signs (BBT, cervical mucus), those provide more precise dating.

When does implantation occur?

Implantation typically happens 6‑12 days after conception. The calculator provides an estimated implantation window.

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