Reverse Gestational Dating

Conception Date Calculator

Find out when you likely conceived. Enter your estimated due date or the start of your last menstrual period to estimate your fertile window and probable day of fertilization.

Reverse Embryonic Calculation Statistical Estimate

Conception Date Calculator

Determine when fertilization likely occurred and discover your estimated conception window.

Range: 20–45 days

Calibrates estimated ovulation relative to standard 28-day cycle length.

How Is a Conception Date Estimated?

Human biological conception occurs when a viable sperm fertilizes a mature ovum, which typically takes place within 12 to 24 hours of ovulation. Because embryonic development averages 266 days (38 weeks), we can work backward from an estimated due date:

Estimated Conception Date = Estimated Due Date − 266 Days

Why Is There a Conception “Window”?

While fertilization must happen within about 24 hours of ovulation, sperm can survive inside the female cervical crypts and fallopian tubes for up to 5 days. Therefore, intercourse occurring up to 5 days before ovulation can result in conception on the day of ovulation.

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