How to find your conception date

Wondering exactly when you conceived? There are four ways to work it out — from your due date, your last period, an ultrasound, or your ovulation date. None give a single guaranteed day (conception is a ~6-day window, not a fixed moment), but each gives a useful estimate.

Method 1: From your due date (the 266-day rule)

A pregnancy lasts roughly 266 days from conception (38 weeks), compared to 280 days from your last menstrual period. So:

Conception date ≈ Due date − 266 days

If your due date is December 1, 2026, your estimated conception date is around March 10, 2026 (266 days earlier). The actual conception likely happened within a 6-day window around that date — 5 days before plus the day of ovulation.

Method 2: From your last menstrual period (LMP)

For a textbook 28-day cycle, ovulation (and therefore conception) happens around day 14 — two weeks after the first day of your last period.

Conception date ≈ First day of LMP + 14 days (for a 28-day cycle)

If your cycle isn't 28 days, adjust: ovulation happens roughly 14 days before your next period would start. So for a 32-day cycle, ovulation is around day 18; for a 24-day cycle, around day 10.

Ovulation day ≈ Cycle length − 14

Method 3: From an early ultrasound

A first-trimester dating ultrasound (weeks 7-13) measures the embryo's crown-rump length (CRL), which grows at a remarkably consistent rate. The scan gives a gestational age in weeks and days — and conception happened approximately 2 weeks after that gestational age start point (because pregnancy is dated from LMP, and ovulation is ~2 weeks after LMP).

So if a scan at exactly 9 weeks gestation is done on May 1, the conception date is around March 13 (9 weeks back minus the 2-week LMP offset = ~7 weeks before the scan).

Ultrasound dating is the most accurate method when the scan is done early — if it differs from your LMP-based estimate by more than 7 days in the first trimester, ACOG guidelines say the scan dating should be used.

Method 4: From your ovulation date (most precise — if you tracked it)

If you tracked ovulation with OPK strips, basal body temperature, or cervical mucus, you have the most precise estimate: conception happened on or within a day of your ovulation date. The egg is only viable for 12-24 hours after release, so fertilisation must occur in that window (sperm that arrived in the preceding 5 days can wait).

See our guide on signs of ovulation to learn how to track it for next time.

Why your conception date is always an estimate

Even with perfect data, conception can't be pinned to a single day for most pregnancies. Reasons:

• The fertile window is ~6 days — 5 days before ovulation plus the day of
• Sperm survive up to 5 days — intercourse on day 9 could result in conception on day 14
• Ovulation timing varies — even with regular cycles, the exact day can shift 1-3 days
• Implantation isn't conception — fertilisation happens within a day of ovulation; implantation is 6-12 days later

For paternity questions, courts and clinics typically use a 10-14 day conception window rather than a single date — and DNA testing is the only definitive answer.

Quick reference table