Bloodwork to determine baby's sex?

I'm hearing from more people that they did bloodwork to determine if baby was a boy or girl. I'm just not  sure why if that's an option, do people have the ultrasounds which aren't as accurate? Anyone know about this? 
Husband wants to find out like tomorrow and I keep telling him it's way too early for ultrasound or bloodwork lol. But we're obviously excited to find out!!