Twin pregnancy

I’m currently pregnant with twins and had my first ultrasound 2 days ago. baby a is measuring on time at 9w1d with a fetal heartbeat of 171, but baby b is only measuring at 7w4d and they couldn’t find a fetal heartbeat. They only did a pelvic ultrasound not a transvaginal. They are in separate sacs. They told me that baby b isn’t viable. I have hope that at the next ultrasound baby b grows and they can find a heartbeat. Has this happened to anyone else and had a good outcome?