5 weeks transabdominal ultrasound with only gestational sac

I had my TAU yesterday but my OB only seen a gestational sac without the yolk and fetal pole LMP: March 6,2019 been having this brown spotting and lower back manageable pain... and referred me for a transvaginal utz next week... has someone experience this and eventually ending up with a viable pregnancy??? I have 2previous blighted ovum @6weeks and 8weeks when my spotting turned to fresh blood. then blessed with a baby girl then a fetal death in utero last 2015 then now Were trying to have this... It really makes me worry?? 6months in the making then a positive pt and now this... im nauseated, craving positive, headache, sleepiness, fatigue, joint pains, ...Im taking duphaston now once a day for 10days... thanks for any advice