If pregnancy is rounded off to 9 months then why do doctors give your due date on the 40th week/10th month?

in school, you always learn women are pregnant for 9 months
but then why do doctors say your due date is your 10th month? going off a 4 week month....

and many women give brith past due date.
Update: when some of you are telling me its because its the whole nine months, im not understanding what your saying...
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