Some people accept biology (specifically the stages of human development) while other people deny it.
That doesn't change any biological truths with regard to human development. That does not change the fact that, from the very moment of fertilization ("conception"), the resulting human zygote is ever-striving to continue through all the stages of human development (this 'striving' can only occur while life is present... no life? no further development occurs). I will reference what Wikipedia lists as 'stages of human development' (from the moment of fertilization):
Zygote
Embryo
Fetus
Infant
Toddler
Child
Preadolescent
Adolescent
Emerging/Early Adulthood
Young Adult
Middle Adult
Old Adult
Dying
A 'toddler' that lacks life is no longer striving to reach the 'child' stage of human development. A 'young adult' that lacks life is no longer striving to reach the 'old adult' stage of human development. A 'fetus' that lacks life is no longer striving to reach the 'infant' stage of human development.
To kill an 'old adult' is to kill a living human. Likewise, to kill an 'embryo' is to kill a living human. From the moment of fertilization (formation of a zygote), the resulting zygote contains all genetic material necessary to develop through every single stage of human life. (zygote --> embryo --> fetus --> newborn, ... ... ... elderly adult)
Those are all biological truths. You can either accept them (be rational) or deny them (be irrational).