Due Date by Ovulation

Estimate a due date from an ovulation date or a conception date.

Last updated: 9/28/2026

If you know when you ovulated or conceived, date the pregnancy from that day instead of from a period. Add 266 days (38 weeks). The standard period method adds 280 days because it starts about two weeks earlier.

Calculator Inputs

Ovulation or conception date*

This date is*

Results

Enter values above to see your results

These dates are estimates for planning, not a medical diagnosis. An ultrasound is more accurate, and only a small share of babies are born on the exact due date.

How to use this calculator

Enter the ovulation date or switch the label if the date you have is conception. For a natural cycle those two dates are usually within a day, and this tool uses the same 266-day count for both.
Read the due date, the estimated last-period date (14 days earlier), and how many weeks pregnant that implies today.
If the pregnancy started with an embryo transfer, do not use this page. Transfer dating depends on the embryo’s age, which this calculator does not ask for.

How the calculation works

Clinical gestational age counts from the last menstrual period, not from conception. In a 28-day cycle, ovulation is treated as day 14, so the due date is 280 − 14 = 266 days after ovulation.

The example date, 14 March 2026, plus 266 days is 5 December 2026. The implied period start is 14 days earlier, 28 February 2026. On the due date the gestational age is 40 weeks and 0 days.

Today’s week count is the number of days since ovulation, plus 14, then divided by 7. That extra 14 days is the obstetric clock, not extra time the pregnancy has existed.

When to use ovulation dating instead of the period calculator

The period calculator is the better default when you know the first day of bleeding and your usual cycle length. Use this page when ovulation was observed (test strip, temperature shift, monitored cycle) or when a clinician gave you a conception date.

A long or short cycle moves ovulation away from day 14. Adding 280 days to the period in that case is less accurate than adding 266 days to a known ovulation. If you only know the period, the pregnancy due date calculator already shifts the result by the difference from a 28-day cycle.

Worked example

Ovulation on 14 March 2026

Due date:14 Mar 2026 + 266 days= 5 Dec 2026

Implied period:14 Mar 2026 − 14 days= 28 Feb 2026

Age at due date:266 + 14 days= 40 weeks 0 days

Frequently Asked Questions

How is a due date calculated from ovulation?

Add 266 days to the ovulation date. That is 38 weeks after ovulation and about 40 weeks after the period that would have started 14 days earlier.

Is conception the same as ovulation?

Not exactly. Conception is fertilization, which happens in the hours after the egg is released. For a due-date estimate the difference is usually one day or less, so both dates use the same 266-day rule here.

Why does this disagree with a period-based due date?

The period method assumes ovulation on day 14. If you ovulated earlier or later, the two results will differ by that same number of days. Ultrasound dating, when you have it, replaces both estimates.

Related guides

Guides & Articles

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Due Date From Ovulation or Conception Date

Add 266 days to an ovulation date or a conception date. A worked example, why both dates use the same rule, and when the period method is the better start.

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How Accurate Are Pregnancy Due Dates?

A due date is the 40-week mark from a period, or 38 weeks from ovulation. Most births are not on that exact day. What moves the estimate, and what replaces it.

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How Many Days Is a Pregnancy?

280 days (40 weeks) from the last menstrual period, or 266 days (38 weeks) from conception. Same model, different starting point, plus the term labels clinicians use.

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