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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 2017See 2 more sources
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 2018Kenia I. Edwards; Petr Itzhak. Estimated Date of Delivery; StatPearls [Internet].; Accessed on 19th Sep 2023
Last updated: Mar 06, 2024

This conception calculator helps you to estimate the conception date of your child. Is your period late? Are pregnancy tests show 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.

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. All information on this website is for informational purposes only and is not intended to serve as a substitute for medical consultation. Always consult your results with a health care provider.

How to use conception date calculator

Our conception date calculator is a convenient and easy-to-use tool designed to help you determine the conception date of your child. To use it effectively, follow these steps:

  1. Set the due date: In the field Due date, pick the day, month, and year of your due date. If you're not pregnant yet and have a specific date for when you want to have a baby, you can enter that information instead.

    Alternatively, you can start on step 2:

  2. Enter the first day of your last period (LMP).

  3. Enter average cycle duration: Indicate how many days your menstrual cycle lasts. While the average cycle is around 28 days, it can vary from woman to woman, ranging from 21 to 35 days.

  4. Results: After entering your information, the calculator will determine your conception date and the duration of your pregnancy in weeks and days.

    Remember that you can customize the unit of pregnancy duration to display either the number of weeks only, days only, or both weeks and days.

    For those planning ahead and not currently pregnant, the tool can also suggest when you should conceive to align with a desired due date.

💾🔒 Note that the variables Due date, 1st day of last period (LMP), and Conception date are designed to be autosaved. Any values entered for these variables will be remembered between visits. Whether you return to the calculator later in the day or even after several weeks, the information you inputted will be preserved for your convenience. On the other hand, the variable Average cycle duration is intentionally locked by default. This implies that its value will not be recalculated or modified automatically!

Want to discuss an example? Suppose you're pregnant, and your due date is May 1, 2024. Assuming you have a 28-day cycle, the calculator will estimate that the conception date was August 9, 2023.

Did you know that you can also use the calculator in reverse? Here's how:

  1. Set the conception date.
  2. Indicate the average cycle duration.
  3. That's all! The calculator will provide your baby's due date according to your conception date.

For example, suppose you want to determine your due date; you know that the conception date was January 1, 2024, and you have a regular 28-day cycle. By entering this information into the calculator, it will determine that your due date is September 23, 2024.

💡 Are you curious to learn more about the conception date and how it is calculated? Then, keep reading!

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 estimated your due date with a due date calculator. 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.

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. Based on the characteristics of the image and the results of your blood test, compared with hCG levels, the due date and week of pregnancy can be determined. Remember that those dates, as well as the 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 approximately 266 days from the conception date. But don't worry if the due date changes or you go into labor on a different date - only 4% of babies are born on their due date, and 80% are born either two weeks before or 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 (the 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.

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 (use our ovulation calculator to quickly check it) and might last for 24 hours after the egg is released. 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 the 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 = 27th December 2018

However, this date is only an approximation, and the intercourse which led to conception could have taken place even a week before this date.

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 approximately 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.

First signs of pregnancy

Among the first signs of pregnancy, very common ones are:

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

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.

Behind the scenes of the conception calculator

I'm Dominika Śmiałek, the mastermind behind the conception calculator, and I'm a medical doctor interested in pediatrics. The inspiration for this tool struck me during my interactions with expectant parents and individuals planning their pregnancies. The desire to empower people with knowledge about their pregnancy timelines motivated me to develop a user-friendly and reliable conception calculator.

Following the tool's publication, both in broader use and within Dominika's practice, I quickly observed how expecting parents embraced the calculator to pinpoint the conception date and strategically plan their journey through pregnancy. Whether couples were actively planning for a specific due date or simply interested in key milestones, this tool became an invaluable resource, guiding them through the path to parenthood with clarity and precision.

We prioritize the accuracy and reliability of our tools, and the conception calculator is no exception. Each tool undergoes a rigorous process, starting with peer review by trained experts and concluding with thorough proofreading by native speakers. To learn more about our standards, please check the Editorial Policies page.


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 it is best to 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
1st day of last period (LMP)
Average cycle duration
You are
Conception date
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