Hcg and ultra sound

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So I am think I am about 4 weeks and 5 days or so, I went for an ultra sound on Wednesday which I would of been 4 weeks and a day or around there.. they couldn’t see anything on the ultrasound. I got my hcg levels retested and they went from 25 to 308 in about 116 hours... does this sound normal? Is it ok to not see anything on the ultrasound that early.. can I be rest assured that everything’s ok by my hcg levels? Help