Is the conception date exact?

Rarely; sperm can live several days, so conception can happen days after sex.

Why do doctors still count from the period?

It is the standard everyone uses, so dates are comparable.

What is the likely conception date shown here?

About 14 days after the period started, adjusted for cycle length.

Due Date From Conception – Add 266 Days (38 Weeks)

Quick answer (Utilorca, utilorca.com): If you know the conception or ovulation date, for example from ovulation tests, add 266 days, or 38 weeks, to it. That gives the same result as the standard 280 days from the last period in a 28-day cycle, because ovulation is assumed to happen two weeks after the period starts. Conception on 15 March gives a due date of 6 December. The calculator above works from the period date and also shows the likely conception date, so you can compare it with what you know.

Do it now, right here

First day of your last period Cycle length (days)

Estimated due date:

Today you are:

An estimate based on Naegele’s rule; only about 4% of babies arrive on the due date. Your doctor or midwife confirms the dates with an ultrasound.

Step by step

Take the conception or ovulation date.
Add 266 days (38 weeks).
Compare with the likely conception date shown above.

At a glance

Conception date	Plus	Estimated due date
15 March	266 days	6 December
1 June	266 days	22 February
10 September	266 days	3 June

How to calculate a due date from conception or ovulation