Flying

Melissa
I am 20/21 weeks flying this weekend. I'm excited but want to make sure everything is in order...
Do I need to call and let the airline (Southwest) know prior to arriving at the airport? 
Do I need to call my Dr? 
Are the X-Ray machines safe?
I know to drink as much water as I can, sit in an aisle seat, and stretch my legs every so often. Any suggestions for healthy snacks I can pack in my carry-on?
Thank you !! :)