Every month doesn't have just 28 days (equals 4 weeks) there are 2-3 more days per month. Those days are what makes it 9 months. From your first day of your last period you will be due 9 months and 7 days from that day. So you can say its a little over 9 months. BUT you aren't pregnant until you ovulate, which normally occurs 14 days after your first day of your period. So in all reality, you are only pregnant for 9 months, sometimes a little more if you go past your due date and sometimes less if you go before your due date.
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Ps I asked my wifes obstretician lol