Thinking about the abortion debate, I have determined that I believe that life begins with the formation of a diploid zygote. In sexual reproduction (which includes artificial insemination; essentailly, sexual reproduction means all forms of human reproduction other than cloning) this occurs at karyogamy. (In cloning, it occurs at whatever point the cell becomes capable of mitosis, or, I think, just after the electric shock stimulating the inserted nucleus to restrart the cell cycle).
In any case, this means in essence that what defiens contraception as pre- or post-conception for me is whether or not it precents karyogamy. I am uncomfortable with IUDs because they prevent implantation of the egg rather than preventing the sperm from meeting the egg. I am also not entirely comfortable with the Pill or with emergency contraception because I have been under the impression that in some cases it prevents implantation rather than ovulation. On the other hand, some studies have suggested that emergency contraception only works by preventing ovulation, and does not prevent implantation which would make me quite comfortable with it (and with the pill i the same is true of it).
However, I will go a step further and say that I have no problem with preventing implantation if doing so prevents karyogamy. I know that the sperm unites with a secondary oocyte, which goes on to develop into an egg through meiosis II. If this process can be stopped before the oocyte becomes an ovum and before the nuclei merge, I have no problem with it.
Of course, I have problems with artificial conception techniques (e.g. invitro fertilization) when they produce large number of embryos that are simply discarded.
That may not be all, but it is all for now.