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10 Month or 9 Month Pregnancy

Everyone knows that a pregnancy lasts 9 months right? I mean we all know that a pregnancy consists of 3 trimesters over a 40 week period – right? But 40 weeks – with 4 weeks in a month isn’t that 10 months?

For those that live by the Lunar calendar, this not only makes sense, but is also correct. The Lunar calendar, which is the four weeks it takes for the moon to go from a new moon, to a full moon and back again, would make a pregnancy last for ten months
For those that live by the Gregorian calendar, and counting by the date you started your last period (let’s say October 15), you would not be one full month pregnant until November 15 and the calculations generally look like this:

The answer is actually quite simple. We all know that pregnancies cover a period of 40 weeks, and as you can see from above if you consider 4 weeks to be a month then you would be correct in saying that a pregnancy covers 10 months. On average though there are actually 4.33 weeks in every month according to the Gregorian calendar that the world goes by. However, neither of these calculations is actually accurate.   

The way your health care provider will calculate your due date by doing the following:
Add nine months and seven days to the date which you started your period (LMP). This would be about 7 days if you were on a 28 day cycle. This means that your due date would be the 22 July and not the 15 July as you would expect – and this is 40 weeks exactly.
So, if we use this method, then your pregnancy would look like this