Hcg not doubling

So I posted about this earlier, but Tuesday night hcg levels were 1400 and this morning they were 2400. The doctor said he’s leaning towards a miscarriage because they aren’t doubling even though it had not yet been 48 hours. Then he told me it’s not a good pregnancy til he hear a heart beat I’m absolutely terrified now. I thought it went up pretty good from Tuesday night and I thought it was every 48-72 hours. And then I got an ultra sound and he said he “thinks” it’s a gestational sac. Anyone have any opinions. I’m scared now. And I’m either 4 or 5 weeks, 5 based off my lmp bit I think I ovulated late.