How a Reverse Due Date Calculator Works

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When you think about pregnancy calculators, you probably don’t often consider ones that count in reverse, but a reverse pregnancy calculator is one that counts backward from your due date to the date when you conceived or may want to try to conceive for a certain due date range.

This information is most useful to someone trying to plan a birth for a certain date range. The calculator offers an idea of when to have sex in order to achieve a particular due date.

Why Use a Reverse Due Date Calculator?

A reverse due date calculator might be useful for a variety of reasons. As noted above, If you want to have your baby in, say, early summer, the reverse due date calculator gives you a window in which you would need to conceive to target that prospective due date.

Note that there is a lot of luck and chance involved with trying for a preferred due date. Also, babies are not likely to be born on their exact due date, for example, and it's not guaranteed that you will conceive during your preferred cycle—even if you have sex on the right days and your fertility is optimal.

An expectant parent also might use a reverser due date calculator if they are curious about the date of conception.

Predicting Due Date

To understand how the reverse due date calculation works, it is helpful to consider how “forward” due dates work too.

The way we calculate due dates is typically not based on the actual date of conception, but rather an educated guess. The date is usually predicted to be 280 days from the first day of your last menstrual cycle. Pregnancy lasts, on average, 266 days. Doctors just tack 14 days onto that date to account for the first day of your last menstrual cycle through when an average day of ovulation occurs—which on average may be day 14.

However, some women who track ovulation know exactly when they ovulated. This makes the 280 version of a due date less precise. In those cases, it's more exact to use the 266-day rule and tack that on to the date of ovulation. So, if you ovulate on day 10, you’d have a due date of 276 days; if you ovulate at day 20, you’d have a 286-day due date.

Calculating a Reverse Due Date

When you look in reverse the due date becomes a bit harder to pinpoint, particularly if you have no idea when you got pregnant. But, this method starts with the due date (either the one given to you by your doctor or your preferred date if you're not already pregnant).

The method is simple in theory—you take the due date and subtract 266 days. The reason this is tricky is that you’re not really doing one equation of numbers. You have to figure it out by months and partial months. Essentially, starting with 266, you sequentially subtract away each month (or partial month) until you arrive at the likely date of conception.

Here is an example using the due date of December 8th:

  • December 8 minus 8 days in December = 258 (November 30th)
  • 258 minus 30 days in November = 228 (October 31st)
  • 228 minus 31 days in October = 197 (September 30th)
  • 197 minus 30 days in September = 167 (August 31st)
  • 167 minus 31 days in August = 136 (July 31st)
  • 136 minus 31 days in July = 105 (June 30th)
  • 105 minus 30 days in June = 75 (May 31st)
  • 75 minus 31 days in May = 44 (April 30th)
  • 44 minus 30 days in April = 14 (March 31st)
  • 14 minus 14 days would be March 15th.

To double-check this date, you can put your first day of the last period (in the above example, using the 280-day method, we’d guess that to be about March 1st) into a pregnancy due date calculator. If you've done your math correctly, you should get the due date (or close to it) that you started with.

Drawbacks to Reverse Due Date Calculations

The main problem with finding your reverse due date is simply having a math error, which might put your conception date off a bit. Additionally, your actual date of conception might be a few days before or after the one you calculate simply due to when you ovulate and have sex within your cycle.

If you're already pregnant and unsure about your due date (if the date of your last menstrual period is unknown), then it will be more difficult to accurately calculate your reverse due date as well. If you’re guessing on your dates, then early ultrasound, ​and the earlier the better, will be the most accurate way to establish your due date.

The egg will only last about 24 hours after ovulation, so the timing of conception is really quite a narrow window from the standpoint of the egg. So, what is more in play is the length of time sperm can live in the female’s body, and that can be from four to seven days depending on the conditions.

In the example above using a December 8 due date, the pregnant person may have gotten pregnant on March 15th. Alternatively, they may have just as easily gotten pregnant from having sex on March 10th, 12th, 16th, or another day close to the 15th.

So, it would be safe to say that the window of possible conception for the due date of December 8th would be: March 7th to 17th. You, individually, could narrow this down further by calculating when you knew you had sex or when you didn’t.

A Word From Verywell

While it can be fun to figure out a conception date or to try to pick your baby’s due date, establishing a reverse due date is definitely not an exact science that will reveal your specific date of conception. However, a reverse due date calculation will get you close, whether you are already pregnant or hoping to conceive at a specific time.

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