I count from after 3 months, ie 1 month 4 weeks, end of 3 months is 13 weeks
I was like you. Than i decided to follow this as reference. Lol...
12weeks + 1day onwards. After 12 weeks, ie. The 13th week.
I followed 14+1 is second trimester. 3rd is 28+1.
After end of week 12. Which is week 13 onwards
I count from 13 weeks
13 weeks onwards
Week 13 onwards
Wk12
13 weeks 1day