HELP! Mixed feelings so nervous

I had my first prenatal visit today.. according to my period I should have been 8 weeks 5 days.. while getting a ultrasound my doctor tells me I'm pregnant with TWINS!! Seeing 2 sacs but she can only see 1 baby.. according to the ultrasound I'm measuring at 6 weeks 2 days so she said it was too early to detect a heartbeat.. I'm getting my blood drawn for my hcg levels and another ultrasound in 2 weeks... has this happened to anyone and had a positive outcome?? I'm so nervous! Thank u