5 Ways to Calculate Your Estimated Conception Date

Dominique W. Brooks
Pregnant woman in her third trimester

Once you get pregnant, it is often difficult to determine your exact date of conception. Because most women don't know when they ovulated, they can only estimate when their baby was conceived. However, estimating your conception date may be useful when trying to determine your due date or even simply to assess your baby's growth at this stage in your pregnancy. There are several ways you can make an educated guess.

Figuring out the Date of Conception

There are a few methods for figuring out your date of conception. Naegele's Rule is a standard approach to determine the baby's due date that is popular among physicians. Once the expected delivery date (EDD) is established, you can estimate the date of conception.

Naegele's Rule

Naegele's Rule is a calculation that many obstetricians use to estimate the possible delivery date. The method requires you to know the first day of your last period (LMP). The calculation is as follows:

  • Determine the LMP.
  • Date back three months.
  • Add seven days.

Once you figure out your due date, you can count back 38 weeks, which should give you an approximate date of conception. These two relatively simple calculations carry some assumptions that may not be accurate for everyone. Assumptions include the following:

  • The pregnancy lasts 40 weeks.
  • The patient has a 28-day cycle.
  • Ovulation occurs two weeks after conception.

There have also been some questions about the number of days that are added on -- some suggest 10 instead of 7 -- because this may set the delivery date too early. This is still being discussed and evaluated so as for now, the rule has not been changed.

Ultrasound Estimates

If you're wondering "When did I conceive?", based on ultrasound, doctors can estimate. Another way to estimate your conception date is through an ultrasound. Ultrasounds are usually performed at some point between weeks eight and 20 of pregnancy. They are used to confirm gestational age of a pregnancy. Gestational age is based on your last missed period. The age of the fetus (known as fetal age) is the age of your child since conception. Gestational age should always be one to two weeks older than fetal age.

To figure out the gestational age of the child, doctors and radiologists take measurements of the baby's legs, arms, head, and torso. Based on these measurements, doctors can give an estimated gestational age of your child. Subtract two weeks to find the fetal age, and then count back in the calendar to figure out your estimated conception date. Ultrasounds can only provide estimates of gestational age. Some babies are smaller or larger than the average. Earlier ultrasounds tend to be more accurate. Overall, however, they can be up to two weeks off on estimation.

LMP Method

The LMP (last menstrual period) method of estimating when you conceived is how some doctors figure your conception date and due date. For women who have a regular 28-day cycle, ovulation often occurs on or around day 14 according to the Mayo Clinic.

Most women assume that their conception occurred around mid-cycle. However, if you have irregular cycles, you can use other methods of determining conception dates.

Basal Temperature Method

The Mayo Clinic also says if you have been keeping track of your basal body temperature, you may know the length of your luteal phase--otherwise known as days past ovulation (DPO). A slight rise in the body temperature indicates that ovulation has occurred. The days between ovulation and the day before your next period are considered the luteal phase. Even if cycles are irregular, DPO usually stays the same. Therefore, you can count back using your DPO to begin estimating conception date.

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.

To use the calculator above:

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

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

Can You Tell When It Happens?

Some women say they knew they were pregnant the moment it happened. For them, the question "When did I conceive?" seems easy. They talk about a feeling of excitement, of certainty, even of completeness. It is possible that some women can tell. It's also possible that these women thought they "knew" they were pregnant at other times too, and then forgot about that feeling when the pregnancy test was negative.

About 20 percent of women have physical symptoms when they ovulate. These women feel a sharp or crampy pain, called mittleschmerz, on one side of the pelvis around the time that an egg is released. If you have mittleschmerz, you'll be able to guess your conception date within a few days.

Conception Dates Are Only Estimates

Conception dates are only considered estimates for several reasons. First of all, even women with regular cycles may not ovulate on day 14 each month. It can vary slightly, anywhere from day 12 to day 16 for a regular woman on a 28-day cycle. Other reasons conception dates are just estimated include the following:

  • Sperm life can be up to three to five days.
  • Stress or other life changes can change even regular women's cycles for that month.
  • Eggs take 12 to 24 hours to be shed, so the estimated conception date may be off by one day.
  • Spotting at implantation can be mistaken for a last missed period, throwing off conception date by as much as a month.

The only completely accurate way of knowing conception date is when infertility treatments, such as artificial insemination or in vitro fertilization treatments have been used.

Does It Matter?

Although you may never know exactly when you conceived, it can be fun and useful to try to pinpoint the range of days that it could have occurred. Once the baby is born, the conception date won't matter for most women and you can spend your time enjoying your new little one!

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5 Ways to Calculate Your Estimated Conception Date