gestational age vs fetal age and genetic testing

According to my LMP, I am 10 weeks pregnant. When I was 8 weeks pregnant, I went and got a private ultrasound done, as my doctor did not date my pregnancy or confirm a heartbeat at my first appointment. The ultrasound tech told me that I was only 6 weeks (referring to fetal age) and that I'd have to come back in two weeks to hear the heartbeat better. I went yesterday and again a different tech told me that I am no where near 10 weeks and that I am closer to 8. I'm so confused. Do I got by gestational age or fetal age? When I go to my appointment at 12 weeks to get my genetic testing, I will really only be 10 weeks which the tech said was too little to get the test done. Help! What do I do? Call my doctor to ask her to date my pregnancy? Just go by gestational age?