Conception Calculator — Estimate Your Conception Date

How Conception Date Is Estimated

Conception occurs when a sperm successfully fertilizes an egg. The fertilized egg (zygote) then travels down the fallopian tube over several days before implanting in the uterine lining, approximately 6–10 days after fertilization. This is the moment a pregnancy biologically begins, though the medical convention dates pregnancy from the last menstrual period (LMP), which precedes ovulation and conception by about two weeks.

Working Backward from a Due Date

The standard gestational age is 40 weeks (280 days) counted from the first day of the last menstrual period. Since ovulation and conception typically occur around day 14 of a 28-day cycle, conception is approximately 266 days (38 weeks) before the due date.

Why Conception Date Is Difficult to Pinpoint

• Sperm can survive in the female reproductive tract for up to 5 days, so conception can occur days after intercourse
• Cycle length varies — in a 32-day cycle, ovulation occurs around day 18, not day 14
• Fertilization itself is microscopic and undetectable without medical testing
• The earliest a pregnancy can be confirmed is after implantation (when hCG rises enough to detect)

Medical Dating vs. Calendar Dating

A first-trimester ultrasound (usually between 8–13 weeks) is the most accurate way to establish gestational age. It measures the crown-rump length and can predict the due date within ±5 days — more accurately than calendar calculations, which assume a perfectly regular cycle.