You may already know the drill, or if not you, certainly your sister, cousin, or friend does. Whenever a woman gets a positive pregnancy test at the doctor’s office, the first thing the clinician will usually do is grab a pregnancy wheel from the drawer while asking her the first day of her last menstrual period. Entering that one date gives women their due date. But how accurate are these wheels?

The problem is, they are based on the assumption that women ovulate on Day 14, which may or may not be true for you personally, or may not have been the case for that particular cycle. In essence, the wheel measures the gestational age of your pregnancy. But a more accurate approach is to determine the fetal age, which is measured from the day of conception.

So rather than relying on Day 14 averages for your due date, if you are charting, there is a simple mathematical formula for accurately predicting your due date based on when you actually ovulated that cycle:

Add 9 months to the day of your thermal shift

and then subtract 7 days from that date.

So, for example, if your temperature shift was on April 20, you would jump ahead to January 20 and subtract 7 days, for an approximate due date of January 13. If you actually did ovulate about Day 14, the estimated due date would be about the same for both the formula and pregnancy wheel. But if you ovulated well after Day 14, the formula would be substantially more accurate.

Of course, let’s be honest, even if the due date is based on when you actually ovulated, the baby is going to come when the baby is going to come. They just do that as a way of preparing you for when they grow into teenagers.