Two sacs, could only see 1 baby

Hey all mommies and mommies to be. I'm currently freaking out. My fiancé has the twin gene and was teasing me about it because I was showing a little sooner than I should. I'm 8 weeks today and day before yesterday had some cramping and light spotting so we went to the hospital. Our first ultrasound and md said baby A is growing and has a strong heartbeat at 164. But baby B could not be found, just the sac. This was an external ultrasound. So we are just hoping that baby is there but hiding. Because the md said if there is no baby in sac B my body may try to expel it and take health baby A with it. Any other mamas had this happen?