Safely into 2nd trimester?

At what point do you feel like you are safely into your second trimester?
For example, I don't know how accurate a sonogram is in telling you "you are
8 weeks along today" etc, but if this makes sense I am wondering when you
can officially feel you are in your 2nd trimester?

If my doctor wants me to try to wait until 2nd trimester for certain
medications it's not like I want to jump into them the day I reach 13 weeks.
But more than that I was just wondering if certain developments take place
in 1st trimester and not counting date or days,. you are officially in the
2nd trimester after a developmental stage etc..this may be a silly question!