I tested on 13 dpo. I kept thinking about a buddy's late BFP, it was partly what kept me using the progesterone cream. I was thinking that it was possible that somewhere inside me was a little fertilized egg that was trying to find a place to implant and it was up to me to try to help improve the environment with the progesterone (I've tested low for progesterone twice).

While studies are inconclusive about progesterone preventing miscarriage, there is still a lot they haven't studied and some clinical evidence points to progesterone supplementation actually helping "rescue" the corpus lutuem. If your progesterone production is lame and implantation can happen late then there is a chance that supplementing progesterone can keep the endometrium in a state that the egg can implant whereas without it the environment is inhospitable.