Blood test for sex of baby?

Maggie • 25🌞Mommy to💙Luca 12/31/13💙 Logan 02/14/17💙
Hey everyone. I feel pretty dumb posting this considering this is baby #2 but I didn't have this done with my first and wanted to learn more about it.
What week is the test done? I'm assuming it's also tied into a few other tests for genetic disorders/abnormalities, correct? 
I had blood tests done with my 1st in 2013 but I didn't find out the sex that way so I was wondering if it's normal for doctors to do that or if you have to ask for it. 
Thanks in advance! 😊