Find out the sex with a blood test?

Kelsey • Wife of Nate since June 29, 2012. After two miscarriages in 2012, our son, Alex was born January 24, 2016
I've never heard of this but at my 9 week appt today my OB said that anytime after 10 weeks we can do a blood test to find out the sex of the baby! I didn't know that was possible! I thought you had to wait until you could see on an ultrasound?