Twins 1 low heart beat & measuring 5 days behind

Katy

Hello me and my husband booked in for an early scan (7wks+1) she found identical twins (same sac) twin A heart rate was 133bpm and measured 6weeks +4 but twin B was measuring 5weeks+6 with just 55bpm. She basically said to us it is very likely the other one won't survive. Has anyone else been through anything similar? Just wondered if there was any change twin B will survive x