Conception Calculator

Created by Dominika Śmiałek, MD, PhD candidate
Reviewed by Dominik Czernia, PhD and Jack Bowater
Based on research by
Committee Opinion No 700: Methods for Estimating the Due Date. Obstetrics & Gynecology (May 2017)See 1 more source
Kessler J, Johnsen SL, Ebbing C, Karlsen HO, Rasmussen S, Kiserud T. Estimated date of delivery based on second trimester fetal head circumference: A population-based validation of 21 451 deliveries. Acta Obstetricia et Gynecologica Scandinavica (October 2018)
Last updated: Mar 10, 2022

This conception calculator helps you to estimate the conception date of your child. Is your period late? Are pregnancy tests shows clear results? Congratulations! Or perhaps you're not pregnant yet, but want to have a kid on an exact date? Don't hesitate - the pregnancy conception date calculator also works in reverse - it tells you when to conceive if you want a child born on a day of your choosing.

With our conception calendar calculator, not only do you get an answer to the question when did I conceive and how many weeks am I pregnant?, but you can also learn a bit about when does conception happen, and how long does conception take.

Conception date - when did I conceive and how many weeks am I?

Do you remember the moment you realized you were pregnant? After the initial excitement, you probably wanted to know the due date. And after that you might have gotten curious about the day you conceived. This conception calendar calculator will help you put all your doubts aside.

We know that having children changes your life completely. It affects your daily plans, holidays, and overall free time. Some say that it's easier for a kid to be the oldest in their class - they tend to be more mature and gain new knowledge easier. That's only one of the reasons you might want to have your child's birthday thought out well ahead. Although whether mother nature will allow you to keep such plans is another story, you can still use this pregnancy conception calculator to estimate when you should conceive to have a kid on a certain date.

Conception calculator based on due date

So, what day did I conceive? There are two ways to use our conception calendar calculator.

The first way is to provide the birth due date. When you visit the doctor they will most likely perform an ultrasound. Although the doctor can see the baby from the 5-6th week of pregnancy, the best time to go for the first sonography is between week 8 and 12.

Based on the characteristics of the image and the results of your blood test, the due date and week of pregnancy can be determined. Remember that those dates, as well as conception calendar, are only an estimation and may change during the next visit.

Pregnancy typically lasts around 40 weeks (counted from the first day of your last period). That is 266 days from the conception date. If a baby is born before week 37, it's called a premature birth. But don't worry if the due date changes or you go in labor on a different date - only 4% of babies are born on their due date and 80% are born either two weeks before and after the date.

Let's go back to the question: what day did I conceive? In our conception calculator, we subtract 266 days from the estimated due date and get the most probable day of conception:

due date - 266 = conception date

Conception calculator based on LMP

The other method of using the pregnancy conception calculator is to provide the first day of your last menstruation (LMP) and the average length of your menstrual cycle. The majority of cycles last around 28 days. However, typical values range between 21 and 35 days.

In cycles of 28 days, ovulation happens around 14 days before the beginning of the next cycle. So, if a cycle lasts 28 days, ovulation occurs the 14th day after the first day of the cycle (first day of bleeding). If it's shorter, you subtract the corresponding number, e.g., for a 22 days cycle - ovulation occurs on the 8th day. If it's longer, you add some days, e.g., for a 34 day cycle - ovulation is on the 20th day. For cycles longer than 35 days or shorter than 21 days, these calculations are not applicable and you should consult by a doctor. If you're not sure how long your cycles are, leave the default number of 28 days.

When does conception happen? How long does conception take? The fertile window begins 3-5 days before the ovulation date, and ends one day after it. It means, to get pregnant, you should have intercourse within this time. Although sperm can survive up to 7 days inside a woman's body, conception does not happen until ovulation. Therefore, conception day is the day of ovulation and might last for 24 hours after the egg is release. Therefore, the formula to estimate the conception date based on the LMP:

LMP + cycle length - 14 = conception date

Practical example of the use of conception date calculator

Let's take Anne. She is 11 weeks pregnant, and still doesn't know her conception day. The first day of her last period (LMP) was 11 June 2018, and typically, her cycle lasts for 31 days (still in range, between 21-35). As mentioned before, we can assume that her ovulation began around 17 days after her LMP, on 28 June 2018.

11th June 2018 + 17 days = 28th June 2018

She conceived around her ovulation date (as a woman's egg can last up to 24 hours), on 28 June 2018. However, instead of all this counting, she used our pregnancy conception calculator.

Next up is Maggie. She just came back from the doctor, who told her that the due date is on 19 September 2019. Now, she has one question: 'what day did I conceive?'.

19th September 2019 - 266 days = 26th January 2019

You don't have to do the calculations by hand! Just put the numbers into our conception date calculator based on the due date. It can also work as a pregnancy week calculator, giving you an estimated week of pregnancy and the first day of your last period.

Now, to ensure that both the mother and child are healthy during and after pregnancy, both Anne and Maggie should control their pregnancy weight gain.

Can the pregnancy conception calculator be wrong?

Yes, of course. So, when does conception happen? There is no possibility being 100 % sure about it. You may be a little bit more certain if you track your cycle and have it perfectly regular. But even ultrasounds and blood tests aren't faultless. Remember, that your fertile window begins up to 5 days before ovulation and ends one day after it. So, although your conception date is, e.g., 11 July 2019, the intercourse that caused the pregnancy may have occurred as early as 6 July. However, with our conception calendar calculator, you can make an estimated guess!

First signs of pregnancy

Is your period late? Do you have some of the symptoms listed below?

  • Tender, swollen breasts
  • Morning nausea
  • Food aversions or cravings
  • Constipation
  • Mood swings

You might be pregnant! Congratulations. Now it's time to take a pregnancy test and make an appointment with a doctor.

I am pregnant, what next?

Congratulations again! A new journey in your life begins. Although you can't know everything, and a lot of things might change, you can predict some things, such as your child's height, or even their blood type!

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.


How accurate are conception dates?

No conception date calculator is completely accurate, as they are estimating the date it occurred. The most accurate measure is to take an ultrasound and have a professional estimate the age of your baby for you. This service, however, will likely cost you though, so use best use an online calculator.

Does alcohol affect conception and implantation?

Moderate drinking does not impact fertility or the risk of miscarriage in regular pregnancy, a number of studies have found. However, if you are trying for a baby using IVF, both partners should not consume any alcohol. The woman should always stop drinking as soon as she finds out she is pregnant.

Where does conception occur?

Conception occurs in the fallopian tubes, more specifically the ampulla, where the oviduct curves around the ovary. The egg releases large amounts of progesterone here, which the sperm are attracted to. This is how the sperm know how to reach the egg.

When should I take a pregnancy test calculator based on conception?

You can estimate the day you conceived on as soon as you know you have missed your period. This is also the best day to take a pregnancy test. Use Omni Calculator to work out your conception date quickly and easily.

What is conception date?

Conception date is the day on which a baby was conceived, that is, the day on which the father’s sperm fertilised the mother’s egg.

How far along am I based on conception?

  1. Calculate the number of full month differences between today’s date and your conception date.
  2. Calculate the number of days in each of the months in step 1.
  3. Work out the number of days from the conception date until the end of that month.
  4. Find the number of days from the beginning of this month until today.
  5. Add together the result of step 2, 3 and 4. This is how far along you are.

When is my conception date?

To find your conception date, you will have to perform some calculations to estimate it. Find out on what day your last period began, and add your average cycle length in days to that date. Finally, subtract 14 days and you will have an estimate of conception date.

Dominika Śmiałek, MD, PhD candidate
To calculate the conception date, please enter the due date or the first day of your last period (LMP).
Due date
First day of your last period (LMP)
My average cycle duration
I am in week
of my pregnancy
I conceived on
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