[Pill]

I was under the impression that the morning after pill prevented implantation of a zygote into the uterine wall. I'd have to look it up and double-check, though.

[edit]
After reading around some, I see that the current thinking is indeed that prevention of ovulation is the primary mechanism by which EC pills prevent pregnancy.


Your theory relies upon two points: That ECPs will only delay ovulation for a day, and that an ovum released post-coitus could travel all the way down the fallopian tube before sperm from that same coital act could reach it.

The first is hypothetically possible, and probably depends on the specific ECP used, but I am fairly sure that the second is false, so I do not think that under those circumstances ECPs could cause a pregnancy that would not otherwise happen.

Of course, I could be wrong; I'd sooner hear it from someone who knows more.