Conception calculator

Work backwards to the likely conception date — and the ±fertile window around it — from a period date, a doctor’s due date, or an ultrasound estimate.

How to Conception calculator

1. Step 1. Choose your reference: last period, due date, or ultrasound.
2. Step 2. Enter the date (and gestational age at scan, if using ultrasound).
3. Step 3. Adjust cycle length if it differs from 28 days.
4. Step 4. Review the estimated conception date and fertile window.

Frequently asked questions

How is conception estimated?

Conception (fertilization) typically occurs within 24 hours of ovulation, about 14 days before the next expected period — day 14 of a 28-day cycle.

Can conception date be exact?

No — sperm survive up to 5 days and ovulation timing varies, so results are windows, not certainties. Early ultrasound narrows it to ±3–5 days.

Why does cycle length matter?

The luteal phase (ovulation to period) is fairly fixed at ~14 days; longer cycles usually mean later ovulation, shifting conception later.