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Due Date from Conception Calculator

Enter the conception date to estimate the due date.

Result

2026-12-06
2026-03-15
2026-03-01

How it is calculated

The tool takes the conception date you enter and does simple calendar arithmetic. Due date = conception date + 266 days, which is 38 weeks, the length of pregnancy counted from fertilization. It also shows estimated LMP = conception date − 14 days, the first day of the last menstrual period that would correspond to that conception, which is the date doctors usually quote for 40-week dating. Both dates are written as year-month-day. The tool does not use your cycle length, and it cannot correct for an uncertain conception date. A date that does not exist in the calendar, such as 30 February, is quietly moved to the next valid day instead of being rejected. Ultrasound dating and your own doctor's estimate always take precedence.

Worked example

Suppose conception is documented as 2026-03-15, for example from an IVF transfer record. Due date: 15 March + 266 days. The rest of March gives 16 days, then April to November add 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 = 244, which makes 260, and 6 more days lead to 2026-12-06. The estimated LMP is 15 March − 14 days = 2026-03-01. Counting 280 days from that LMP gives the same due date, 2026-12-06, so the two systems agree.

Try this example →

Common mistakes

  • Entering the first day of the last period instead of the conception date. That date belongs in an LMP-based calculator; here it would put the due date about two weeks too early.
  • Using the date of intercourse as the conception date when sperm can survive for days before fertilization. Ovulation or embryo-transfer records give a better date.
  • Typing an impossible date such as 2026-02-30. The tool does not reject it but moves it to 2026-03-02, so the result is for a different date than you meant.
  • Treating the due date as the day of birth. It is a single day in a range, and a normal delivery can occur weeks before or after it.
  • Writing the date as day-month-year with slashes (15/03/2026). Use year-month-day (2026-03-15) to avoid it being read the wrong way.

Sources

About this calculator

When the conception date is known — for example from ovulation tracking or IVF — the estimated due date (EDD) is calculated as conception date + 266 days (38 weeks), which reflects the actual length of pregnancy from fertilization. This differs from the more common Naegele's rule, which adds 280 days to the last menstrual period and assumes conception occurred about two weeks after that date.

Expecting parents who tracked ovulation, used fertility apps, or conceived via IVF (where the exact conception or transfer date is documented) use this calculator for a more precise due-date estimate than a standard LMP-based calculation, and it also back-calculates the corresponding last menstrual period date for comparison with a midwife's or doctor's records.

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Comments (1)

  • Editorial team

    With embryo transfers, the transfer date is not the conception date. Clinics generally count back by the embryo's age at transfer, such as three or five days, before using a calculation like this one. Enter the date you end up with. Your fertility clinic's or doctor's own due date takes precedence over any online estimate. Which source did your conception date come from?

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