HCG levels drastically slowing after 6 weeks

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HCG levels drastically slowing after 6 weeks

Should I be concerned with these levels? I read HCG levels slow down after a level of 6000 is reached however I wasn’t expecting such a drastic slow down. I had two 5 day blastocysts transferred so maybe a vanishing twin?
6/18 - 538 (12 days post transfer)
6/20 - 1518
6/25 - 8187
6/29 - 9811 (6 weeks)
7/2 - 13251

Any insight would be greatly appreciated.

Thanks!
Beth