hcg levels are rising but not doubling

Ca • Tue, Oct 17

I'm 6 weeks and my hcg levels are 42,458 as of yesterday ... Thursday they were 24,000 Saturday 34,977 and yesterday 42,458 they are rising but not doubling. had ultrasound at 6 weeks no visible heart so said threatened miscarriage... I've heard from everyone 6 weeks not everyone sees a heart. I am Confused 😔 anyone go thru this ?