Weeks 1-4 you are in your 1st month
Weeks 4-8 = 2nd month
Weeks 9-13 = 3rd month
Weeks 14-17 = 4th month
Weeks 18-21 = 5th month
Weeks 22-26 = 6th month
Weeks 27-30 = 7th month
Weeks 31-35 = 8th month
Weeks 36-40 = 9th month
https://www.baby2see.com/pregnancylength.html