12 weeks= 3 months? or the date baby was born every month?

Brittany
so i'm confused. app says baby is 12 weeks which i assume is 3 months. but my friend said since my baby was born on the 16th of the month that every 16th is when i add a month. so baby turned 12 weeks on the 9th is that when he is considered 3 months or is it on the 16th? 

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