Dont know how many of you ladies have been to this site.
But pampers.com
has a wonderful week by week pregnancy calendar....
They show actual pictures of what your baby should look like from week to week and give you information about each particular week of development.
OH and per their website for the ladies that are 26 weeks or about to be 26 weeks it says THAT is the start of the 3rd trimester!
Here is the link
Pampers Pregnancy Calendar