Firstly, I operate on a moral premise that all living humans have certain inalienable rights; including the right to not-be-killed.
When defining whether or not an unborn child qualifies, I go by the post-birth murder test. As in, what are the requirements of the victim for the death to be considered as murder?;
1) The organism has to be alive (initially). Definitions of life vary (usually a list of processes such as reproductive capacity, metabolise its environment, autonomous respirations etc.). In order to be consistent, the definition of life has to be applicable to all living organisms.
2) The organism has to be human life; i.e. human offspring (or genetically human) – a human form (or subcategory) of life.
3) The organism has to be a unique living human – as in, not merely a part of a human (this can also be genetically determined if necessary).
If such an organism is killed (without rational justification), then a murder has been committed. If any one of these criteria are absent in the victim, the act is not murder. All of these criteria exist in an unborn human child.
Conception is the most objective starting point for human life. Haploid cells (i.e. egg & sperm) have no potential to mature into an adult human, but a diploid embryo has everything it needs (given the right conditions) to live a full human life. The embryo/zygote/foetus is therefore a unique human life with the same value and rights as all other unique human lives.
To deny an unborn child the right to not-be-killed requires an appeal to subsequent logical entities such as the nebulous concept of person-hood, or reliance on the activity of some preferred vital organ. These are completely subjective criteria – motivated by a desire to justify abortion (often under the guise of women’s rights).
All morality has some element of subjectivity from a finite human perspective. Nevertheless, surely a civilized society would err on the side of life, rather than potential infanticide.