How a Reverse Due Date Calculator Works

Pregnancy Wheel

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When you think about pregnancy calculators, you probably don’t often think about ones that count backward, but a backward pregnancy calculator is one that counts backward from your due date to the date where you conceived. While it may be hard to pinpoint the day you actually had sex, it can narrow down a window of time where conception was likely to happen.

Forward Due Date Prediction Method

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. We 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—which may be day 14.

The thing is many women know exactly when they ovulated. This makes the 280 version of a due date a bit on the useless side. But it does work well 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. But that’s the forward-looking mechanism.

Reverse Due Date Prediction

When you look in reverse it becomes even harder to pinpoint, particularly if you have no idea when you got pregnant. But, it starts with the due date. You can use one of the handy gestation wheels to see where it marks two weeks pregnant, but many moms don’t happen to have these “pregnancy wheels” laying around. So it involves a tiny bit of math.

Basically, you take the due date and subtract 266 days. The reason this is tricky is that you’re not really talking a simple equation of numbers. You have to figure it out slowly by months and partial months.

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 days – 14 would be March 15th.

Now, to double-check this date, you can go put your first day of the last period (using the 280-day method, we’d guess that to be about March 1st) into a pregnancy due date calculator. That also tells you the date you should have a positive pregnancy test, feeling the baby move and hear the heartbeat. You can usually get good guesstimates if everything was near or before those dates.

What Might Be off With the Reverse Due Date Calculation

This might be off and how off will depend on many factors, including how the due date you used, to begin with, was calculated. Sometimes your doctor or midwife will calculate it based on a guess of when you had your last period. Sometimes it is based on an early ultrasound. If you’re guessing on your dates, then early ultrasound, ​and the earlier the better, will be the most accurate.

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. But nature really wanted us to reproduce, 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.

So if you smiled above and assumed our fictitious woman got pregnant on March 15th, you may be right, but she may have also gotten pregnant from having sex on March 10th. Also, if this woman ovulates earlier in her cycle, she may have gotten pregnant slightly later than March 14th. So it would be safe to say that the window of possible conception for the due date of December 8th would be: March 7th – 17th. You, individually, could narrow this further by calculating in when you knew you had sex or when you didn’t.

Either way, this is a fun little calculation to play around with, and it shows off the intricacies of the human body. So instead of asking when is your due date, I’ll ask, when did you conceive?

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