Hcg doubling every 72 hrs instead of 48 hrs...vanishing twin?

So I've had 3 betas over the last week:

14dpiui: 104

18 dpiui: 285

20 dpiui: 450

Initially my numbers were doubling every 66hrs which falls within the normal range of 48-72 hours. But now it has slowed to doubling every 72.8 hrs...right outside the normal guidelines. Any hope this could be the result of a vanishing twin? I had 2 perfect follicles this iui and I got a positive digital hpt at 10dpiui so maybe there were 2 embryos to start and now just 1 left? Or do you think this is ectopic? Any advice or stories are greatly appreciated!