It is in our best interests to value conscious persons and all compassionate people do. But such consciousness does not exist without the brain architecture necessary to generate it, particularly a functioning cerebral cortex. That brain architecture does not develop in a fetus until the beginning of the third trimester of pregnancy at the earliest. Before that time a fetus has no more personality or sentience than lower-order animals. Indeed until a month after conception, a fetus doesn't even have a brain.
Accordingly, if an abortion can be procured without harm to the mother before the third trimester then it does no significant harm at all, and may even bring about positive good (such as by allowing a family to provide more resources to the children they have rather than dividing those resources further; by reducing the devastating effects of overpopulation; by ensuring the health and continued happiness of the mother; or by avoiding subjecting a future child to a bad parenting environment). But after the third trimester has begun, unless the fetus has failed to develop a functioning cerebral cortex, elective abortion is physically equivalent to infanticide and is thus reprehensible to anyone who values conscious persons, and would be permissible to outlaw, although abortion to save the life of the mother is always an acceptable act of self-defense.