Conception Calculator

Estimated Conception Date
Tue, 20 Jan, 2026
Likely Range: Sun, 18 Jan, 2026 – Thu, 22 Jan, 2026
Estimated Due Date
Tue, 13 Oct, 2026

When did conception happen?

Conception usually happens around ovulation, about two weeks after the first day of the last period. Working back from the due date, it is about 266 days (38 weeks) earlier. This calculator estimates the likely date and a range of a few days either side.

How the conception date is estimated

Conception ≈ due date − 266 days, or first day of last period + 14 days

Pregnancy is counted from the last period, about two weeks before conception, so a pregnancy of 40 weeks lasts about 38 weeks from conception. The likely range is two days either side, because the egg can be fertilised up to a day after ovulation and sperm can survive several days before it.

Good to know

  • The date is an estimate. Ovulation timing varies, so conception can easily be a few days either side.
  • A due date from an early ultrasound (before 13 weeks) gives the most reliable conception estimate.
  • After IVF, the conception date is known: the day of egg retrieval, or the transfer date minus the embryo's age.
  • A conception date can't establish paternity with certainty, because the possible window spans several days. Only a DNA test can.
  • This calculator gives estimates for information only and is not a substitute for advice from a doctor.

Worked example

  1. Due date given by the doctor: 15 June 2027
  2. Conception: 15 June 2027 − 266 days = around 22 September 2026
  3. Likely range: 20 to 24 September 2026
  4. Working from a last period that started on 1 October 2026 instead, conception was around 15 October 2026, with a due date of 8 July 2027

Conception date by due date

Estimated conception dates for due dates through 2027.

Due dateLikely conceptionRange
1 January 202710 April 20268 April 2026 – 12 April 2026
1 March 20278 June 20266 June 2026 – 10 June 2026
1 May 20278 August 20266 August 2026 – 10 August 2026
1 July 20278 October 20266 October 2026 – 10 October 2026
1 September 20279 December 20267 December 2026 – 11 December 2026
1 November 20278 February 20276 February 2027 – 10 February 2027

Each date is 266 days before the due date, with two days either side.

Frequently Asked Questions

Subtract 266 days (38 weeks) from your due date, or add about 14 days to the first day of your last period if your cycle is 28 days.
It gives the likely date within a few days. Ovulation can vary, and sperm can survive for several days, so the exact day can't be pinned down from dates alone.
Doctors count pregnancy from the first day of the last period, which is about two weeks before conception. At conception you are already counted as about two weeks pregnant.
No. The possible window covers several days, so dates alone can't prove paternity. Only a DNA test can.

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