5 Ways to Calculate Your Estimated Conception Date

While no method is 100% accurate, you can try a few different ones to come up with a good estimate.

Updated April 28, 2023
pregnant couple looking at calendar

Knowing your conception date won't affect much about your pregnancy. You'll still be able to figure out your due date through other methods, and there's no quiz at the end of your pregnancy that will require you to know the answer. Still, it's something we're all curious about, especially if the pregnancy is unexpected.

While no method of calculating your date of conception is completely accurate, these can help you make an educated estimate.

Conception Dates 101

Your conception date, also known as a fertilization date, is the day that your baby was conceived. It is the moment that occurs after ovulation when a sperm fertilizes an egg. About 6 days later, the embryo implants into the uterine lining and grows into a developing fetus. Conception dates are only estimates, and they may vary based on the method used to calculate them.

Use a Conception Date Calculator

One of the fastest and most convenient ways to estimate the date of conception is with an online widget calculator like the one below. To use this method, you will need to know the first day of your last period and the average length of your menstrual cycle.

To use the calculator:

  • Choose the average number of days between periods in the first box.
  • Select the month, day and year of your last period in the following boxes.
  • Click the "Calculate" button.

To perform a new calculation, click the "Clear" button after your first calculation.

Naegele's Rule

Naegele's Rule uses the first day of your last menstrual period (LMP) and adds 280 days to it. German obstetrician Franz Naegele developed this method in the 19th century to provide an estimation of a pregnant person's due date. Many doctors use this formula as a guideline to determine the estimated due date (EDD) of your pregnancy.

Though Naegele's method is primarily used to provide your due date, it can also estimate your conception date by subtracting 266 days (38 weeks) from the due date.

Using this method, the formula to calculate your date of conception (and remember, this is a ballpark estimate) is:

First day of your last period + 280 days - 266 days = Estimated day of conception

Last Menstrual Period (LMP)

The last menstrual period (LMP) method involves using the first day of a person's last menstrual period to estimate the conception date. To use this method, you will need to know the first day of your LMP and count backwards by 14 days.

This method is not always accurate, because it assumes that everyone ovulates 14 days after their first menstrual period, which is not true. The length of menstrual cycles and timing of ovulation can vary from person to person.

Basal Body Temperature (BBT) Method

The basal body temperature (BBT) method involves tracking your BBT over time to determine when ovulation occurred. BBT is the lowest body temperature that a person experiences in a 24-hour period. It is typically measured first thing in the morning before getting out of bed. BBT is lower during the first half of a person's menstrual cycle, when estrogen levels are high, and higher during the second half of the cycle, when progesterone levels are high.

To use this method, you will need to take your BBT every morning before getting out of bed and record your daily temperature on a BBT chart. Many people who are trying to get pregnant may already do this. You can then look for patterns in the temperature changes to determine when ovulation occurred and estimate your conception date.

However, it is important to note that the BBT method is not always accurate and should not be the sole method for determining conception date.

Ultrasound Estimate

A pregnancy dating ultrasound is an imaging scan that is performed during pregnancy to determine the gestational age of the baby and the estimated due date for delivery. They typically perform dating ultrasounds between 8 and 14 weeks of pregnancy.

During a dating ultrasound, your doctor or an ultrasound technician will use a device called a transducer to send high-frequency sound waves through your uterus. These sound waves create an image of the inside of the uterus, which is displayed on a screen. Your doctor will use the images from the scan to measure the size of the fetus and determine how far along your pregnancy is.

Once you know your baby's gestational age, subtract two weeks to estimate your date of conception. Dating ultrasounds provide accurate measurements of your growing baby, but it is important to note that due dates and conception dates estimated your ultrasound are just that - estimates.

Can You Tell When Conception Occurs?

Some people say they knew when they were pregnant the moment conception occurred. People report feelings of "just knowing" with certainty. While this may be true, being aware of exactly when conception occurs is not common.

That said, some people know when they ovulate. Up to 40% of people who ovulate experience ovulation pain, called mittelschmerz. They usually describe this as a sharp, pinching, or cramping pain on one side of the lower abdomen around the time ovaries release an egg. If you know exactly when you ovulated, it may be easier to guess your conception date within a day or two.

Conception Dates Are Only Estimates

For most people, conception dates are only estimates, because a number of factors can affect the timing of conception.

  • Sperm: Sperm can live inside the female reproductive tract for up to five days. This means that conception can occur several days after sexual intercourse, making it difficult to pinpoint the exact date of conception.
  • Menstrual cycle length: The average menstrual cycle is 28 days long, but this varies from person to person and even from month to month in the same person. This variation can make it difficult to pinpoint the exact date of ovulation.
  • Timing of ovulation: Ovulation typically occurs around the 14th day of the menstrual cycle, but can vary from person to person. For example, some people may ovulate on day 12 of their menstrual cycle, and others may not ovulate until day 17.

Does Conception Date Matter?

Ultimately, the exact date of conception does not matter. Conception dates are only estimates, and they may vary based on the method used to calculate them. Due dates are more important, and even that is an estimate based on the baby's size. Every pregnancy is different, and it is always best to consult with a healthcare provider for accurate information and guidance on having a healthy pregnancy.

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5 Ways to Calculate Your Estimated Conception Date