Blighted Ovum? Or?

Recently on Monday, I went for a scan requested by the doctor that did my IUI. Gestational sac has grown however he says there’s no visible yolk sac. If counting contraception date of 20 July, I should be about 6 weeks now. However a gynae I’m also separately seeing had already arrange for a scan 26 Aug which would be when we should see something. I’m confused and concerned. Was I supposed to already see something on Monday or is it blighted ovum as what my other doctor is insinuating?