How to Calculate Your Conception Date

Two ways to estimate a conception date

The estimate comes from the standard pregnancy-dating convention. It assumes ovulation — and so conception — about 14 days after the first day of the last period, which places it 266 days before the estimated due date. You can estimate a conception date from either date:

For the same pregnancy, both calculations give the same estimated date.

Where the 14 and 266 come from

By convention, the estimated due date is 280 days after the first day of the last menstrual period, and the convention assumes a regular 28-day cycle with ovulation on day 14.¹ Putting those together:

• 14 days — the convention’s assumed time from the first day of the last period to ovulation.
• 266 days — 280 − 14, the remaining time from that assumed ovulation to the due date (38 weeks).

The 266-day figure is derived from those two conventions rather than stated directly as a clinical rule. That’s why this site always labels the result as an estimate by convention. The 280-day rule itself is explained in how a due date is calculated.

How the last period, conception and due date fit together

All three dates sit on one timeline, counted in days from the first day of the last period (day zero):²

14 + 266 = 280: the two calculations above are just two ways of moving along this timeline.

How to work it out yourself

1. Take your estimated due date. Example: 15 April 2027.
2. Count back 266 days (38 weeks) on the calendar.
3. The date you land on — here, 23 July 2026 — is your estimated conception date by convention.

Counting back 38 weeks is straightforward: the same weekday, 38 weeks earlier. But a calculator avoids slips across months of different lengths.

A second example, starting from the period: if your last period began on 2 March 2026, adding 14 days gives an estimated conception date of 16 March 2026, and the estimated due date is 7 December 2026. Counting back 266 days from 7 December 2026 lands on the same 16 March.

Why it can’t be exact

The convention doesn’t account for irregular cycle length or variation in when ovulation happens,¹ and real pregnancies vary too. In a study that identified ovulation with hormone tests, the time from ovulation to birth had a median of 268 days, and pregnancy length varied over a 37-day range even after excluding preterm births.³

Those two facts together explain the limit: the calculation uses a fixed 266 days, while real pregnancies don’t all last the same length of time. A calculator has no way to know where in that range your pregnancy falls, so it can’t determine the exact date conception happened. If your cycles are irregular, read about due dates with irregular periods.

Does a scan tell you when you conceived?

Ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is.¹ That result is given as gestational age, counted from the first day of the last period rather than from conception.² If your due date was set after a scan, using that due date gives a conventional estimate that matches it — but it is still an estimate. Read more about how scan dating works.

How conception fits with weeks and the due date

Because pregnancy is counted from the last period,² the conventional estimate always falls at 2 weeks 0 days of the pregnancy. In the example:

• 9 July 2026 — first day of the last period (0 weeks 0 days)
• 23 July 2026 — estimated conception date by convention (2 weeks 0 days)
• 15 April 2027 — estimated due date (40 weeks 0 days)

To see how far along you are today, you can find out how many weeks pregnant you are.

What about IVF?

With IVF, the due date is assigned from the embryo age and transfer date — 261 days after transfer for a day-5 embryo and 263 days for a day-3 embryo — and that ART-derived date should be used.¹

These figures line up with the 266-day convention: a day-5 embryo transferred on 21 March 2026 gives a due date of 7 December 2026, and so does a day-3 embryo transferred on 19 March 2026. In both cases, subtracting the embryo’s age from the transfer date points back to 16 March — the same day the convention would place conception for that due date. Only day-3 and day-5 transfers are covered; for anything else, and for your actual treatment dates, your fertility clinic is the best source.

What the calculator result means — and what it doesn’t

Can a conception date establish paternity?

No. An estimated conception date can’t establish paternity and shouldn’t be used to determine it.

Frequently asked questions

When did I conceive if I know my due date?

By convention, about 266 days before it. For a due date of 15 April 2027, that is 23 July 2026 — an estimate, not an exact date.¹

How accurate is the estimate?

It is only as accurate as the convention behind it, which assumes a regular 28-day cycle with ovulation on day 14.¹ Treat it as an approximate date.

Can I calculate my due date from my conception date?

Yes — the same convention in reverse: estimated conception date + 266 days. You can calculate your estimated due date from a conception date.

Why does the estimated conception date fall at 2 weeks pregnant?

Because pregnancy is counted from the last period and the convention places conception 14 days later.¹ 14 days is 2 weeks 0 days.

Can two pregnancies with the same due date have started on different days?

The calculator gives them the same estimate, because it uses a fixed 266 days. Real pregnancies vary in length, though — over a 37-day range in one study even after excluding preterm births³ — so the actual timing can differ.

Why does the calculator say “by convention”?

Because the result comes from the standard dating convention, not from information about your own cycle. The calculator can’t determine the exact date of conception.

Medical statements in this article are linked to the numbered sources listed below: published clinical guidance and peer-reviewed research. It has not been reviewed by a named clinician. Read how content is written in our editorial policy, and see our medical disclaimer.