Pregnancy Conception Calculator

What this pregnancy conception calculator does

This calculator estimates when conception likely happened, based on a due date, last period, or ultrasound. It shows a probable conception window, a wider possible window, and the intercourse windows that could have led to the pregnancy.

Conception timing is an estimate. A due date is calculated from a 266-day interval, and sperm can survive for several days, so the possible window is wider than a single date.

How to use this calculator

1. Choose due date, last period, or ultrasound.
2. Enter the relevant date.
3. Add the cycle length for a last-period estimate, or the gestational age for an ultrasound.
4. Click Calculate to see the probable and possible windows.

The default example uses today's date as the last period and a 28-day cycle.

Conception window formulas

Conception date = due date − 266 days
Probable conception = conception date ± 2 days
Possible conception = conception date ± 5 days
Probable intercourse = conception date − 3 to +2 days
Possible intercourse = conception date − 8 to +5 days

The windows are planning estimates, not a record of when conception occurred.

What the results mean

• Most probable conception: the narrow window around the calculated conception date.
• Possible conception: a wider window that allows for timing variation.
• Intercourse windows: the days on which intercourse could have led to the pregnancy.
• Pregnancy test / next period / due date: related dates for the cycle.

A pregnancy test is most reliable after a missed period. The calculator's test date is an estimate, not a rule for every test.

Assumptions and limitations

The calculator assumes a valid due date and a normal 266-day interval from conception. It does not use ovulation tests, basal body temperature, or ultrasound follicle data. It cannot identify the exact day of conception.

It does not account for assisted reproduction, fertility treatment, or medical factors that change the timeline. Use it as a conversation aid, not as evidence.

Frequently asked questions

Can I know the exact conception date?

Rarely. A due date gives a window, not a precise day. If you tracked ovulation or had an early ultrasound, that information may narrow the window.

Why is the possible window so wide?

Sperm can survive several days, and ovulation timing varies. The wider window reflects that uncertainty.

Does the calculator work for IVF?

This page is designed for natural conception estimates. IVF timing should be calculated from the transfer date and embryo age using a dedicated IVF timeline.

When should I take a pregnancy test?

Follow the test instructions and your clinician's advice. Testing after a missed period is more reliable than testing very early.

Source

References:

• ACOG — Calculating your due date
• NICHD — Pregnancy

Disclaimer

This calculator is for informational and educational purposes only. It is not medical advice or a fertility diagnosis. Discuss conception timing and pregnancy testing with a qualified clinician.