I had a scan at 5w2d and only saw a small gestational sac. At 6w2d we saw a gestational sac, yolk sac and fetal pole measuring 5w6d. No hb at that time. At 7w5d we saw a nice little blob with a perfect hb measuring 7w2d. I don't think you need to worry that you only saw a sac at 5w.
I also have OHSS - my right ovary was 10mm with 2x40mm cysts, 1x30mm cyst and 2x20mm cysts. The next scan, they were stable. Next scan, they had shrunk, although my corpus luteum had turned into a complex, septated cyst that was bleeding. So I was put on progesterone at 8w5d. I've only had 2 episodes of intense pain from my ovary. I haven't been able to wear my regular jeans since 5weeks because the massive ovary was pushing everything up and out. But at this point, things are resolving on their own and my doc isn't worried about it.
Good luck with everything - I know it can be scary.
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