HCG isn't doubling

Hi,

I was super happy with my positive pregnancy test, but I fear it won't last long :-(
On Wednesday evening my hCG was 49.
Friday morning it was 90.
And Monday evening it was only 159.
:-(
So it didn't double in more than 72 hours...
I'm so sad... I know it's early and there's a high chance it might not work out, you just don't expect it to happen to you...
I'm going back to get blood drawn Thursday morning, but I'm dreading it. I hope for the best, of course, but I'm afraid.
My GP said it could happen that it stagnates for a bit and then picks up again, but I don't really find that reassuring, so I don't have much hope.
How strange that you can feel so sad when it's still so early...