Free pregnancy tool · Private

Conception Calculator When Did I Conceive? Estimate From Due Date, Last Period or Birthday

Work out roughly when conception happened using your due date, the first day of your last period or your baby's birthday. This free conception calculator also shows the likely fertile window, key pregnancy dates and how far along you are today.

  • Three ways to calculate
  • Fertile window and timeline
  • Stays on your device

Conception calculator

Calculate from

Use the due date from your scan or doctor if you have one.

Pregnancy is counted from the first day of the last period, so conception usually happens around the end of week 2, close to ovulation.

Estimated conception date

Tuesday, September 22, 2026

Most likely conceived between Sep 17, 2026 and Sep 23, 2026.

Likely intercourse window
Sep 17, 2026 to Sep 23, 2026
Estimated due date
June 15, 2027
First day of last period
September 8, 2026
Today
–

Pregnancy timeline

Fertile windowConceptionTodayDue date

    Features

    A conception calculator that shows the full picture

    Instead of a single date, you get the likely window, the key milestones and a clear explanation of how certain the estimate really is.

    Three starting points

    Calculate from your due date, the first day of your last period, or your baby's birthday and gestational age at birth.

    Realistic fertile window

    See the range of days intercourse most likely happened, since sperm can survive for several days before ovulation.

    Visual timeline

    Follow the pregnancy from last period to due date, with conception, trimesters and today marked clearly.

    Key milestones

    Implantation, a positive test, the end of each trimester and full term, each with its own date.

    Any cycle length

    Cycles from 21 to 45 days are supported, so the estimate fits you rather than a textbook average.

    Completely private

    Everything is calculated in your browser. Nothing is uploaded and no account is needed.

    How to use

    How to use the conception calculator

    1. Choose your starting point

      Pick due date, last period or baby's birthday, whichever you know most accurately.

    2. Enter the date

      Select the date. For a last period, also set your usual cycle length.

    3. Read your estimate

      See the most likely conception date and the window of days when intercourse probably happened.

    4. Explore the timeline

      Check milestones and how many weeks pregnant you are today, then copy the dates if you need them.

    The maths

    How the conception date is calculated

    A typical pregnancy lasts 280 days from the first day of the last period, and conception happens around ovulation, about two weeks after that period starts.

    Conception calculator timeline from last period to conception about two weeks later and due date 40 weeks later
    ExampleIf your due date is June 15, 2027, counting back 266 days gives an estimated conception date of September 22, 2026. The last period would have started two weeks earlier, on September 8, 2026.

    From your due date

    Count back 266 days, or 38 weeks. This works whatever your cycle length, because the due date already reflects it.

    conception = due date − 266 days

    From your last period

    Ovulation usually happens about 14 days before the next period, so add your cycle length and subtract 14.

    conception = LMP + (cycle − 14)

    From a birthday

    Subtract the gestational age at birth, then add back the two weeks before conception that pregnancy dating includes.

    conception = birth − (weeks × 7 − 14)
    Chart

    Due date to conception date chart

    Estimated conception dates for due dates on the 15th of each month in 2027.

    Due dateEstimated conceptionLikely intercourse window
    January 15, 2027April 24, 2026Apr 19 to Apr 25, 2026
    February 15, 2027May 25, 2026May 20 to May 26, 2026
    March 15, 2027June 22, 2026Jun 17 to Jun 23, 2026
    April 15, 2027July 23, 2026Jul 18 to Jul 24, 2026
    May 15, 2027August 22, 2026Aug 17 to Aug 23, 2026
    June 15, 2027September 22, 2026Sep 17 to Sep 23, 2026
    July 15, 2027October 22, 2026Oct 17 to Oct 23, 2026
    August 15, 2027November 22, 2026Nov 17 to Nov 23, 2026
    September 15, 2027December 23, 2026Dec 18 to Dec 24, 2026
    October 15, 2027January 22, 2027Jan 17 to Jan 23, 2027
    November 15, 2027February 22, 2027Feb 17 to Feb 23, 2027
    December 15, 2027March 24, 2027Mar 19 to Mar 25, 2027
    Early pregnancy

    What happens around conception

    Week 1

    Period starts

    Pregnancy is dated from this day, even though conception has not happened yet.

    Week 2

    Ovulation and fertilisation

    An egg is released and can be fertilised for about 12 to 24 hours. Sperm can wait up to 5 days.

    Week 3

    Implantation

    About 6 to 10 days after ovulation, the embryo attaches to the lining of the uterus.

    Week 4 to 5

    Positive test

    Pregnancy hormone rises enough for most home tests to detect it around the time of a missed period.

    Week 8 to 13

    Dating scan

    An early ultrasound gives the most accurate due date, which in turn gives the best conception estimate.

    Week 40

    Due date

    Only a small share of babies arrive on their due date; most are born within a couple of weeks of it.

    FAQ

    Conception calculator FAQ

    How is the conception date calculated?

    From a due date, count back 266 days, which is 38 weeks. From the first day of your last period, add your cycle length and subtract 14 days, because ovulation usually happens about two weeks before the next period. The result is the most likely day of ovulation and conception.

    How accurate is a conception calculator?

    It gives an estimate, usually within several days. Ovulation can shift from cycle to cycle, and sperm can survive for days before an egg is released. A dating ultrasound in the first trimester is the most accurate way to date a pregnancy, typically to within about a week.

    Can a conception date prove who the father is?

    No. Because intercourse several days before ovulation can lead to pregnancy, and ovulation timing varies, dates alone cannot establish paternity. Only a DNA paternity test can do that reliably.

    How long do sperm and eggs survive?

    Sperm can survive in the female reproductive tract for up to about 5 days, while an egg can be fertilised for roughly 12 to 24 hours after ovulation. That is why the fertile window covers about six days.

    When does implantation happen?

    Implantation usually happens about 6 to 10 days after ovulation, when the fertilised egg attaches to the lining of the uterus. Some people notice light spotting around this time.

    Is the conception date the same as the day of intercourse?

    Not necessarily. Fertilisation can happen up to about 5 days after intercourse, so the day you conceived may be a few days later than the day you had sex.

    What if my cycle is not 28 days long?

    Choose your usual cycle length in the calculator. Ovulation tends to happen about 14 days before the next period, so longer cycles mean later ovulation and conception after the period starts.

    Why is pregnancy counted from the last period instead of conception?

    The first day of the last period is a date most people know, while conception is rarely known exactly. By this convention you are already about two weeks pregnant on the day you conceive.

    This calculator gives estimates for general information only. It is not medical advice and cannot determine paternity. Talk to your doctor or midwife about your pregnancy dates.

    Understand your pregnancy dates

    Use the conception calculator to see when your pregnancy most likely began and what lies ahead. For more pregnancy and health tools, explore our free online tools.

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