I've always read that it is produced by the placenta and enters the blood soon after implantation, but I do recall reading a study a couple of years back that said some can leak from the zygote on it's travels down the tubes and you can have a tiny amount in your system before implantation occurs. I THINK it wasn't enough to make it into the blood stream...not sure...would like to find the article again. Then again, I may be recalling what I read incorrectly, and it was a different hormone being produced:
https://www.ncbi.nlm.nih.gov/m/pubmed/9196793/