ConceiveCalc

Conception Date Calculator

Enter your due date — estimate when conception likely occurred.

Due date

Estimated conception date

March 10, 2026

Gestational weeks (to today): —

What you entered

Due date: 2026-12-01

1
Due date
2026-12-01= December 1, 2026
2
Estimated conception
Due date − 266 days (38 weeks)= March 10, 2026

Result

Estimated conception date: March 10, 2026

Based on a due date of 2026-12-01, the estimated conception date is March 10, 2026.

Estimated conception date from a due date

Working backwards, conception is estimated at 266 days before the due date: 280 days of gestational age minus the two weeks between the last period and ovulation.

Due date	Estimated conception date
20 March 2026	27 June 2025
1 June 2026	8 September 2025
1 September 2026	9 December 2025
25 December 2026	3 April 2026
15 January 2027	24 April 2026
10 May 2027	17 August 2026

Dating from a last menstrual period assumes a regular 28-day cycle and ovulation on day 14. An early ultrasound is more accurate, and your midwife or doctor's date is the one to plan around. Conception can occur across a fertile window several days wide, so treat this as the middle of a range rather than a single day.

How conception date is estimated

A typical pregnancy lasts 266 days (38 weeks) from conception, or 280 days (40 weeks) from the last menstrual period (LMP). Subtracting 266 days from the due date gives the estimated conception date. Conception typically occurs around day 14 of the menstrual cycle (ovulation), so LMP is about 2 weeks before conception.

Why the estimate can vary

This calculation assumes a standard 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, conception could have occurred earlier or later. Ultrasound dating, especially in the first trimester, is more accurate than LMP-based calculation for establishing gestational age.

Frequently asked questions

How do I find my conception date?

Subtract 266 days (38 weeks) from your due date, or subtract 14 days from your LMP date. If your due date is March 15, conception was approximately June 8 of the prior year. The actual conception window is ±5 days, since sperm can survive up to 5 days and the egg lives 12–24 hours.

Is the conception date the same as the date I had sex?

Not necessarily. Sperm survive up to 5 days inside the reproductive tract, so sex 1–5 days before ovulation can result in conception on ovulation day. The conception date is when the sperm fertilized the egg (at ovulation), which could be days after intercourse.

Can an ultrasound determine the exact conception date?

A first-trimester ultrasound (6–12 weeks) can estimate gestational age within ±5 days, giving a conception window. It measures crown-rump length, which correlates tightly with gestational age early in pregnancy. Later ultrasounds are less accurate for dating because growth rates vary more between individual babies.