Missing twin?

Reade
Twins runs on both sides of our family (husband and mine) yesterday at an apt the doctor put the doppler on and was very confused. She caught a heartbeat of 150 then moved it and caught a heart beat of 160. Im 17 weeks and have had 6+ ultrasounds because im high risk, I've had 3/d and everything what are the chances of missing an identical twin early in the game?