Slow rising hcg and measuring behind

Bfp on 9dpo

First beta in 16dpo = 440

Nothing on ultrasound

Second beta on 25dpo =3148

U/s showed small gestational sac with yolk sac measuring right at 5w0d (should have been 5w4d based on ovulation not LMP)

Third beta today at 28dpo 6w0d= 5875

U/s still just showing gestational sac and yolk and he didnt measure sac

My doubling times are 73-76 hours and I'm worried, but much happier to see them rise again today. I'll start progesterone tonight as a precaution per OB as he said it cant hurt anything.

Anyone have any experience or positive stories for me? I'll have another beta on wednesday morning.