I just go by my due date. I was due June 29th so may 29th I was 8 months, April 29th 7 months etc.
Not all months have the same amount of days, and some months have 5 weeks. But everyone counts different !^
Just divide the number of weeks you are by 4 to get the number of months.
Literally have this saved ;)
First trimester:
Month one: weeks 1-4
Month two: weeks 5-8
Month three: weeks 9-13
Second trimester:
Month four: weeks 14-17
Month five: weeks 18-21
Month six: weeks 22-26
Third trimester:
Month seven: weeks 27-30
Month eight: weeks 31-35
Month nine: weeks 36-40
I believe when your 16 weeks your 4 months