It cannot affect the pattern, for good or evil. All actions are going to be what they'll be. Abortion is no different then any other action, it has consquences and such. People who try to change the pattern are doing so in vain, even Rand.

"Master of the lightnings, rider on the storm,
wearer of a crown of swords, spinner out of fate.
Who thinks he turns the Wheel of Time,
may learn the truth too late."

It's like the Oracle said in the Matrix, "You didn't come here to make a choice, you came here to understand why you made that choice."

You cannot change who you are. People who think they can "change destiny" are foolish. You will do what you will do, even thinking about changing it is still part of who you are and you were going to do it anyway. Causality. Better to ponder on the why, that is where understanding of the self can be found :p