Due Date Calculator by Conception Date

The 266-day method

The standard due date is 280 days after the first day of the last period, and conception is assumed to happen about 14 days after that day. That leaves 266 days from conception to the due date.¹

Worked example

April 18, 2026Conception date
+266 days38 weeks from conception
=Saturday, January 9, 2027Estimated due date

Intercourse is not the same as conception

Sperm can survive in the body for several days, while an egg can only be fertilized for a short time after ovulation. In a classic study of couples trying to conceive, every pregnancy came from intercourse during the six days ending on the day of ovulation.² So if you know the date of intercourse but not of ovulation, conception may have been up to about five days later. When in doubt, a positive ovulation test date is closer to conception than an intercourse date.

Conception age and pregnancy weeks

Clinicians count pregnancy in gestational weeks, from the first day of the last period. Because conception happens about two weeks into that count, a pregnancy that is 8 weeks along is about 6 weeks after conception.³ Everything on this site, including the week-by-week guide, uses gestational weeks.

Limits of this method

Ovulation timing varies, even in people with regular cycles, so a remembered date can easily be several days off.⁴ For IVF pregnancies, use the IVF due date calculator, which accounts for the embryo’s age at transfer. If you have an early ultrasound, its date is generally the more accurate one.¹

Questions people ask

Is the date we had sex the conception date?

Not necessarily. Pregnancies in one large study came only from intercourse in the six days ending on the day of ovulation, so conception can follow intercourse by up to about five days.²

Why am I 2 weeks pregnant on the day I conceived?

Pregnancy weeks are counted from the first day of the last period, about two weeks before ovulation in a typical cycle. So conception falls at around 2 weeks 0 days, and you are not pregnant during weeks 1 and 2.³

Is a conception-based due date more accurate than a period-based one?

Only if you really know the date, for example from IVF or reliable ovulation tracking. For most people the exact day is uncertain, and an early ultrasound is the most accurate way to date the pregnancy.¹

Calculate your due date from conception