No yolk sac, but there is a heartbeat?

courtney🌻 • 21. Lucas Michael & Liam Scott - 9.25.15
Ok so I recently found out I'm having twins. Baby A is measuring 4.3mm (gestational sac and yolk sac) with a 119bpm. Baby B is measuring 2.7mm (GESTATIONAL SAC ONLY) but with a 105bpm. I'm 5 weeks today. Someone reassure me. I'm starting to scare myself. Could they just have missed the yolk sac? My hcg is at 2109.