Implantation Bleeding & Potential Pregnancy After Intercourse

Question
Hi! I was wondering when implantation bleeding should occur? I ask because I grinded ontop of my boyfriend. He urinated right before this and there was only precum. I don't think the precum made contact with my vagina but I could be wrong. Anyway, about 3-4 days after this, I notice a spot of blood in my underwear. I have very irregular periods, sometimes only four a year. Could I be pregnant? He says that he had not ejaculated in over 48 hours and had urinated several times. Thanks!

Answer
Since you have very irregular periods it is not possible to make a guess about pregnancy from likely dates of ovulation. But from what you mention, I assume that there has been no penetration. Pregnancy in the absence of penetrative sexual intercourse is very very rare, in fact the chances of pregnancy with any form of non penetrative act is so negligible that it can be considered zero.