Always count the full months from the first day of your last period, like you would count the number of years
in your age ie.
in your first year you were 0,
in your
second year you were 1 year old, etc. eg. LMP = January 1st ~ on February 1st you are 1 month pregnant, on March 1st you are 2 months pregnant etc.
Interesting triva: 3 calendar months = 13 weeks. Go on, try it!