Because your technically pregnant for 36 weeks but since there's no for sure way to tell at exactly what time you conceived from your last period to a couple of weeks after your last missed period, they add an extra four weeks
Some months have more than 4 weeks in them
Tjis is how i see it each month has 30 to 31 days so technically each monthis more than 4 weeks but to be honest i dont gwt it eithee lol i just round my months and say my weeks when some one asks
Thats in fact a wonderful question, maybe because at 9 months the baby is fully developed and is just in your stomach for the rest of the time just because....I've no clue