conception calculator
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What is a Conception Calculator

A pregnancy conception calculator can help you determine when your baby was conceived, based on a few different factors such as the first date of your last menstrual period (LMP), the due date of your baby, or when the baby was actually born.

Whichever method you choose to use in order to determine the date of conception, it will give you a range (which will coincide with your fertile period) to go by.

You can input the information into the conception calculator below in order to come up with that time-frame. This is when knowing when your period started is important, especially if you haven’t been to the doctor yet.

Use The Conception Calculator to estimate probable conception dates
from Birth, LMP and Due dates.

These estimated conception dates are based on an average cycle of 28 days.

To use the calculator:
1 Choose either the
Due Date / Birth Date or
the First Day of Your Last Menstrual Period,

2 Enter the date, and
3 Click the 'Calculate' button.

First Day of Last Menstrual Period: 

Probable Fertile Window (Ovulation):  ?

Possible Dates of Conception:  ?

Due Date:   (40 weeks)

These estimated conception dates are based on an average cycle of 28 days, which is what most calculators as based on.

While we are aware that some people have a longer or shorter cycle, the average we use for the sake of considering conception is based on those 28 days.

How Does It Work?

The conception calculator figures the gestational age of the baby to determine the date of conception. Since the exact date of conception is hardly ever known, the conception calculator can use the first day of the last menstrual period (LMP) you have to determine the range of days in which conception would have occurred.

You can also use the conception calculator if you know the due date of your baby. The baby can typically be measured 5 or 6 weeks after your last menstrual period to determine the gestational age of the baby and the approximate due date.

If you know either of these figures, you can input this information into the conception calculator along with the current date to find out the possible days of conception.

Again, the conception calculator bases this off of the average 28-day cycle. If you have a shorter or longer cycle you will have to account for that manually, moving the range over a few days in either direction so to speak.

How is the Conception Date Calculated and What is it Based On?

This process is based on a typical, textbook pregnancy. This means that we calculate it based on a pregnancy that lasts 40 weeks and that the woman had a 28-day cycle.

So if you are using the calculator with the day the baby was born that could skew the results, especially if your baby was born earlier or later (which can happen during any pregnancy).

In order to calculate the estimated conception date, the calculator will use the last menstrual period date, date back three months, and then add seven days to determinate your due date.

With the estimated delivery date, it will count back 38 weeks to give an approximate date of conception.

The conception calculator is great for mothers or expecting mothers who are interested in finding out when their baby was conceived (you know, either that trip to the Islands or when the power was out).

Now there are other calculators that we offer, such as ovulation calculators, implantation calculator, due date calculators, and sexual intercourse timing calculators.

These are useful for women interested in their fertility and determining when they have the best chance of becoming pregnant or when their next menstrual cycle will happen.

Facts about Conception

Women who experience regular period cycles typically conceive about 11-21 days after the first day of their last menstrual period.

Determining the exact date of conception is difficult and so your conception date is mostly an estimate based on your period cycle.

There are however special cases, such as women who are artificially inseminated or use in vitro fertilization, and they are able to know the exact date of their conception.

Surprisingly, there are a lot of people who don’t actually know that you cannot conceive any time during the month.

According to one survey, more than 50% of respondents to a survey thought that this was the case, that no matter when you had intercourse you could get pregnant [1].

This causes many people to actually think they are infertile, when in fact they don’t know that there is a specific fertile period to go by. The only actual time to get pregnant is during ovulation, which is when the egg moves through the fallopian tubes and into the uterus [2].

Calculating based on your Estimated Due Date

Typically, your estimated due date is about 40 weeks from the first of your last menstrual period, but this is just an estimate.

While we consider pregnancy to be 40 weeks, it’s really only about 38 because the two weeks before/during ovulation you aren’t pregnant.

Figuring out your conception date based on your EDD is also an estimation. And while you are given an EDD, a lot of babies are not actually born on that day.

If you are trying to calculate the approximate conception date based on your due date you would take your due date and subtract 266 days. This would bring you back to your rough conception date.

Calculating Based on your LMP

With regards to the Last Menstrual Period or the LMP, it is pretty straightforward to calculate but can be confusing for women who have irregular periods.

Basically, the first day of your menstrual cycle is also the first day of your LMP. This works well if you have a regular cycle and those that are using natural family planning will be very astute to when their periods are anticipated as to when their body is ovulating, when to expect PMS, etc [3].

But most of the time, the first day of the LMP is usually what’s needed and nothing more with regards to that period.

Calculating Based on Ovulation Dates/Fertile Period

You can also get to know your ovulation date by using the Ovulation Calculator. As mentioned above, the exact date will depend on having a regular period which is on average 28 days (from the first day of one period to the first day of your next period).

Since you can only get pregnant during your fertile period, you will have an idea as to when you conceived. Some women can tell when they ovulate, and this gives them a very good indicator as to when they conceived as well.

Women who are in turn with their bodies can notice the subtle changes such as changes in the cervical mucus, heightened sense of smell, breast tenderness, pelvic or lower abdominal pain (some can even tell which side they ovulated from), light spotting, and a heightened sex drive (your body wants you to get pregnant).

Knowing when you ovulate can help you determine when you conceive. As it stands the moment of conception with when the sperm meets and finds it’s way into the egg which happens within 24 hours after ovulation [4].

And remember, sperm can survive inside the female for five to seven days [5], so even then there is still a time-frame in which you could conceive, even if you didn’t have intercourse on that day.

The basal temperature method is another way to figure out the days past ovulation, otherwise known as your luteal phase, which are the days between ovulation and the day before your next period.

Calculating Based on your Ultrasound

Another way to find your estimated date of conception is through an ultrasound. Ultrasounds are typically conducted between 8 and 20 weeks of pregnancy and are used to determine the gestational age of the pregnancy.

What Can Affect Conception Dates

There are many factors that can affect when you can conceive. Sperm only last from three to five days, and ultimately it is important to get that sperm into the woman during ovulation.

However, there are things that affect ovulation time-frames. Even when you generally have a regular cycle, there are situations in life that can make that cycle go awry.

There are also things that can affect conception even if you are having intercourse during your ovulation period.

Your body also has to be healthy, and if it isn’t then you might not actually conceive. When there is a lack of nutrients in the body such as zinc, vitamin D, vitamin B6, it could affect the reproductive hormones, which naturally can affect conception [6].

Another big factor when it comes to conception would be stress. When stress hormones are high, there is a 29 percent lower chance of getting pregnant [7].

So that stressful job, life, arguments, and illness can all wreak havoc on the body and conception. Take a break from the stressors in life and find a way to create a Zen place in order to help you conceive.

Medications are another huge one. Antibiotics especially, as well as other medications, can cause a delay in your ovulation and your cycle, which means that if your cycle has been altered, you won’t be able to accurately estimate when you conceived.

Additionally, eggs typically take from 12 to 24 hours to shed, meaning that the conception date may be off by one day.

Is the conception calculator practical?

The practicality of using this calculator vary from woman to woman. Although it can’t say that a baby was conceived on that specific date with 100% certainty, it can provide a solid prediction.

In addition, in situations where a woman had different partners at a given time and would like to know the father of her baby, using a conception calculator can give her a clue. However, a paternity test will be much more reliable in this case, especially if the woman wasn’t ovulating during those times.

Still, the only way to accurately know the exact conception date is if pregnancy is conceived through treatments such as artificial insemination and in vitro fertilization; otherwise, the conception calculator can only give you an estimated idea of the time your baby was conceived.

Frequently Asked Questions About Conception Dates and Conception Calculators

Is the date given accurate?

It’s an estimate and gives a range based on your fertile period. The only way to know for a fact when you conceived is if you only had intercourse once between your cycles.

Can the conception date determine the father of my child?

It depends on when you were fertile and when you had intercourse. If you had intercourse with multiple partners during that time-frame, it could be either of them and the only way to accurately know is by a DNA test.

Why would I want to use a conception calculator?

As said in the above question, you might want to have a range as to when you actually conceived, especially if you were with multiple partners. You also might want to have an idea as to where you conceived to tell your children later on and embarrass them.

Do you have any other question about your period, ovulation, pregnancy or other related topics? Check out our Frequently Asked Questions page.


  1. Beth Skwarecki via Vitals. The Complicated Science behind When Babies are Conceived. [link]
  2. See above #1
  3. See above #1
  4. Marissa via We Have Kids. When Did I Get Pregnant or Conceive? [link]
  5. See above #4
  6. Leah Fessler via Women’s Health Magazine. 17 Weird Things That Can Mess With Your fertility. [link]
  7. see above #6
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