Parasites share a blood supply, and can potentially harm it's host unintentially. But a tumor is a growth inside the body, parasitic in nature, and is comprised of developing cells. You going to fight for tumors too? Some tumors have hair and teeth, so potentially it could have been a human. And, I said one in three fertilized eggs makes it to the second trimester. Not how many make it to term. But that now you are giving your potential life even worse odds. If one in three attach itself to the uterus, and one in three of those ends in misgarriage you have further lowered the potential for life. But again, potential life is way more important than existing life in your view. Next time you jaunt to your bedroom with a box of kleenex and a playboy, you may want to give that a second thought, hmm? Every sperm... is a potential life.