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Pregnancy Conception Calculator

Estimate when conception likely occurred from your due date, last menstrual period, or an ultrasound — with the probable intercourse window, due date, and last-period date.

Estimated conception date

January 15, 2026

Likely intercourse window
January 10, 2026 – January 15, 2026
Corresponding due date
October 8, 2026
First day of last period
January 1, 2026
Estimated conception dateJanuary 15, 2026View results

This calculator works backward to estimate when you conceived. Enter your due date, the first day of your last period, or an ultrasound measurement, and it returns your most likely conception date, the window during which intercourse could have led to pregnancy, and the matching due date and last-period date.

Formula

Conception is dated to ovulation, then everything else follows:

From last period: conception = last period + (cycle length - 14)
From due date:    conception = due date - 266 days
From ultrasound:  conception = scan date - gestational age + 14 days

Because sperm can survive up to about five days, the likely intercourse window runs from five days before the estimated conception date through that date.

A pregnancy is measured from the last menstrual period, so "how many weeks pregnant" is about two weeks more than the time since conception.

Examples

Due date of October 8, 2026

Conception = October 8 − 266 days = January 15, 2026, matching a last period around January 1, 2026.

Last period on January 1, 2026 with a 32-day cycle

Conception = January 1 + (32 − 14) = January 1 + 18 days = January 19, 2026, later than the 28-day-cycle estimate of January 15.

Frequently asked questions

How is the conception date estimated?
Conception happens at ovulation, which is about 14 days before your next period would have started. From your last period it's roughly the cycle length minus 14 days; from a due date it's the due date minus 266 days (38 weeks); from an ultrasound it's worked back from the measured gestational age.
Is the conception date exact?
No — it's an estimate. The actual moment of fertilization can't be pinpointed because sperm can survive in the body for up to about five days, so intercourse on any day in the fertile window could have led to conception. That's why a window of dates is shown alongside the single best estimate.
What is the difference between conception and implantation?
Conception is fertilization — when sperm meets egg, usually within a day of ovulation. Implantation happens about 6–12 days later, when the fertilized egg attaches to the uterine wall. Pregnancy tests detect hormones produced after implantation.
Why does my cycle length matter?
The last-period method assumes ovulation about 14 days before the next period. If your cycles are longer or shorter than 28 days, ovulation shifts, so the calculator uses your cycle length to place conception more accurately.
Which method is most accurate?
An early ultrasound is generally the most accurate way to date a pregnancy. The last-period method is convenient but depends on regular cycles and good recall; the due-date method simply reverses whatever estimate your provider gave you.

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