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I am not sure if that is true. I believe that it also modifies the uterine lining to prevent implantation of an already fertilized ovum.

I believe you're right, that it does make the uterine lining a hostile environment for implantation. Preventing ovulation wouldn't be the "answer" to unprotected sex... it would be too late for that.

"Plan B works through delaying or preventing ovulation, by interfering with fertilization (inhibiting the movement of the egg or the sperm through the fallopian tube), and may inhibit implantation by altering the lining of the uterus. It is not effective if the process of implantation has begun. Plan B will NOT cause a miscarriage...Pregnancies occurring despite treatment do not have an increased risk of adverse outcome."

quoted from the University of Illinois at Urbana-Champaign

"Plan B" (and other emergency contraceptives containing levonorgestrel) unfortunately has several modes of action, one of which may be abortive (prevention of a fertilized egg from implanting in the uterus). The research here is inconclusive, and anyone who tells you otherwise either does not know the science, or is engaged in what they (on either "side") think is a moral lie. The difference between implantation rates of fertilized ova for using or not using emergency contraception is equivalent within statistical errors. However, if you think an ovum at conception is a human being, then the morally safe practice in the face of this scientific ambiguity is to eschew emergency contraception. But there is another reason to as well...

You see, emergency contraception is actually not that effective! If taken after fertilization, it is, or is very close to, WORTHLESS, and poses adverse health risks. Aside from direct medical studies, this is borne out by the fact that abortion rates do not change at all with the availability of emergency contraception - most women take it far too late after intercourse to have the intended effect.

Abstinence (not merely its intent!) is obviously the most effective form of preventing pregnancy; barring that, continuous chemical contraceptive measures are 99.9% effective at preventing fertilization/ovulation, and have no effect on the uterine lining (so in the rare case that pregnancy still occurs, these pills are not responsible for a subsequent abortion via prevention of implantation).

And for all the parents out there that are putting all bets on abstinence for their children, your warnings unfortunately may have to be more explicit. There is a trend afoot in some groups of young women to preserve their "virginity" while engaging in non-vaginal sexual activity. I don't want to get into the gory details, but, aside from the ever present risks of venereal disease, pregnancy can still occur without vaginal penetration.

A terrible time to be a parent!