I don't know about it being a sin, but there are moral implications.
For me, if a woman falls unexpectedly pregnant and doesn't want a baby, then I see absolutely nothing wrong with her having an early abortion.
The body naturally aborts fetuses, many perfectly healthy, below six weeks about 25% of the time. Many women will abort very early fetuses without even realising they are pregnant.
If it's so common for the body (or God if you're theistically inclined) to abort early fetuses for no apparent reason, what's wrong with a woman electively aborting early for her own reasons?
Past a certain point, when the fetus has developed a CNS and especially if it's capable of surviving outside the womb (post 20 weeks) then I would question the morality of aborting. In my opinion the woman has left it too late to change her mind.