How do u know when u make a different month in pregnancy.

Sometimes confusing!: We usually go by weeks and it starts from the day of your last menstrual period. The normal pregnancy is 37-40 weeks long. The first trimester is considered 1-13 weeks (months 1,2,3); the second trimester is considered 14 - 25 weeks (months 4,5,6); the 3rd trimester is 26-40 weeks (months 7,8,9). Here's a website that illustrates it well: http://www.baby2see.com/pregnancylength.html