It does not have to.
Abortion under "valid" (which would still be an act of killing, if the baby is capable of using the brain and a really heavy choice) circimstances (heavy genetic syndromes for example)
would sort of "filter out" the absolutely most unhealthy, extreme genetic misdevelopments or syndromes, like nature used to do.
Not saying that I endorse it yes or no, but I just want to remind to the fact that mother nature has it's own rules when it comes to extreme bad genetic malfunctions or syndromes, which is to filter it out.
However, we, as humans, can use our intelligence for the better of such cases.
One of the many things I am absolutely against, then it is abortion because the mother is too young. Young parents are producing healthier offspring, which is just a fact. It would be a total unreasonable and crazy move, only encouraged by a society that holds cwrtain "standards" for themselves.