I think the first heartbeats start at around week 6 but its not definite. Have you taken any prenatal vitamins like folate? Folate helps in forming fetal pole.
Yet the fetus is just so small, about a centimeter, it could be hard to notice the pole.
I'm at week 11 and I haven't had my first ultrasound. My doctor scheduled it for week 12.
Usually if something is wrong with the forming of the fetus, body rejects it trough miscarriage. So, even though I know it's a terrifying thing, try not to worry a lot a stress can hurt the baby.
Let us know how it goes.
How scary!! I'm so sorry this is happening to you. While I have not experienced what you're going through, I'd love to be kept updated!!