Prenatal blood tests and gender question

I'm due at the end of May and I was curious if anyone has had blood work done for the genetic testing. My OB asked if I wanted to have them done. This is my 4th and have never done it before. I don't even know if my insurance will cover it. I have tricare. Also how does a blood test of my blood tell the gender of my baby?  I'm seeing posts of women talking about the genetic blood tests and they have found out their little ones gender. Any insight anyone?  Thanks! 😀