R.J.
I forget the actual statistics (haven't looked them up in about a year), but from memory, I believe that the numbers are: The average woman having unprotected sex is pregnant without knowing it 4-7 times per year.
From studies done, the single most common reason, was a miss-match of genes/allelles. All of our genetics have to zipper up, and form a brand new complementary DNA double helix. Sometime lethal combinations are made, other times during the "check" that the body does, the DNA is coding incorrectly (these are big bad mistakes...like DNA that won't code for protein, etc. Small mistakes that are STILL lethal get bipassed at this stage, and terminate later :( :( :( While there are literally hundreds of birth defects that are NOT lethal (downs, spina bifida, hydrocephalus, etc)...there are thousands of defects that we don't ever see except for in a lab, because the body terminates.
Sitting in class, looking at all of these studies and research, 2 things always came to mind:
1) How do ANY of us ever get born in the first place?
2) The mental image of a little spirit standing there with her hands on her hips and an eyebrow raised, tapping her foot saying "Nope. Try again. I am NOT going to have two heads! Excuuuuuuse Me. You want me to be born without bones? I don't THINK so. Start over. Let's get it right this time, shall we people???"
It's possible that the test you took at home was more sensitive, or that your hcg levels were higher...and that the blood test will come back positive. It's also possible that this is one of those times (that we only catch when we're hoping&testing&hoping&testing that the dna just didn't line up correctly. Wait for the bloodtest to come back. We'll cross our fingers for you!!!