BFP but not seeming to stick....

Hi, everyone, let me introduce myself. I am a 30 something, stay at home mom. I am currently trying to have a baby..notice I don't say trying to conceive. I have no problem doing that...just can't seem to stay pregnant. I have done it once sucessfully...and have a beautiful son. So I thought the first two miscarriages were flukes. The first one was a missed m/c at 10 weeks, and the second was 4 weeks and 5 days.

But her I am again, pregnant on the first try, 4 weeks and 4 days to be exact and it seems like it is already doomed. I got several BFPs over the last week, went in for a Beta on Friday and the numbers came back HCG 194 and Progesterone 18, which the nurse said looked promising. I just started getting hopeful but on Saturday I began brown spotting....I took another HPT this morning, and it is still positive but not getting any darker. But I go back to Lab today to get a second blood draw, but will have to wait Monday for the results. Waiting sucks...

I am already preparing myself for the news, but it doesn't make it any easier to take. Why is it so easy for me to get pregnant, but why can't I stay pregnant.