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Pregnancy Conception Calculator

Learn how the day you most likely conceived is worked out from the first day of your last period, your due date or your baby's date of birth, and which days most likely led to pregnancy.

  • Free, no sign-up
  • Updated 1 Oct 2026

Planning a pregnancy?

Maternity cover usually has a waiting period, often from nine months to four years, so the best time to check your health plan is before you conceive.

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How is the conception date estimated?

Conception happens when sperm fertilises an egg, usually within a day of ovulation. In a 28-day cycle, ovulation is about two weeks after the first day of your period, so conception is usually around day 14.

If you knowConception date is about
The first day of your last period14 days later, adjusted for your cycle length
Your due date266 days (38 weeks) before it
Your baby's birth date38 weeks before it, if the baby was born at 40 weeks

For example, if your last period started on 1 Mar 2026 and your cycle is 28 days, you most likely conceived around 15 Mar 2026. Sex between 10 Mar to 15 Mar 2026 most likely led to the pregnancy, and your baby is due on 6 Dec 2026.

Why conception is a range of days

Sperm can survive in the body for up to five days, and an egg lives for about a day after ovulation. So sex in the five days before ovulation, or on the day itself, can lead to pregnancy. The day of sex and the day of conception are often not the same.

What happens after conception

  • The fertilised egg travels down the fallopian tube and divides into a ball of cells.
  • About 6 to 12 days after conception, it implants in the lining of the womb. Some people notice light spotting.
  • The body starts making the pregnancy hormone hCG, which home tests detect.
  • Most home tests give a reliable result from the day your period is due.

How accurate is a conception date?

It is an estimate. Ovulation can come earlier or later than usual, even in regular cycles, which moves the conception date by a few days. An ultrasound scan in the first trimester measures the baby and is the most accurate way to date a pregnancy.

Because the exact day cannot be known, a conception date calculator cannot be used to confirm paternity. Only a DNA test can do that.

Frequently asked questions

1.How do I calculate my conception date?

Add about 14 days to the first day of your last period, adjusting for your cycle length. If you know your due date, count back 266 days.

2.Can I find my conception date from my due date?

Yes. Conception is usually about 38 weeks, or 266 days, before the due date, so count back 266 days from it.

3.Can I work out when I was conceived from my birthday?

Yes, roughly. Count back from your birth date by the length of the pregnancy, less two weeks. If you were born at 40 weeks, that is about 266 days. Most babies are born between 38 and 42 weeks.

4.How soon after conception can I take a pregnancy test?

Most home tests are reliable from the day your period is due, about two weeks after conception. Testing earlier can give a false negative.

5.Can a conception calculator tell who the father is?

No. It only estimates a range of days, and ovulation varies. A DNA test is the only reliable way to confirm paternity.

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