As a tangental debate, suppose a late-term mother-to-be is violently assaulted resulting in the death of her unborn child in a premeditated attack intent on causing this outcome. What specifically is the perpetrator guilty of? If we are asserting that the fetus is not a person, then he cannot be guilty of murder in any degree. Presumably there is a lesser crime of "illegal termination" ?
Lot's of people aren't persons. We have arbitrary lines for most of these things based on age but that doesn't necessarily cover things very well - it's just a rule of thumb.
People under 18 aren't persons in the way that people 18 and over are. Minors have fewer rights and cannot contract. Infants don't vote and can't drive and that's no bad thing. In some cultures you're not any kind of person until you are 8 (meaning you have survived through the worst mortality years that carry off so many children and therefore you are worth investing in).
A fetus is less competent than the average infant in that under most circumstances it doesn't do "living" very well. While an infant requires care and maintenance from the mother (in particular) but another human carer could be substituted, a fetus will require much more intensive care and with generally less favourable outcomes.
Many fertilized ova are not destined become people - they fail at an early stage. They can fail at any stage. They can partially fail. Is a baby born without a brain a person? Should they, having been born, be allowed to naturally die within hours or, having the rights of people should we throw every necessary resource at them to keep it's metabolic processes going for 76 years?
As for the case above if a pre-born child is not a person then whosoever causes the loss of a pre-born child in contravention of the parent-to-be's wishes has committed a serious property crime in addition to an assault on the person of the mother. Or the case could be couched in other legalese justifications. However, the child is the legacy of the parent and if the parent decides they don't want to be represented in the next generation that is something that affects the parent and child directly but might not be the business of the rest of us. If someone else makes that decision against the parent and child then that is a violation of the parents will to reproduce. A child is a ward of the parent but the parent may be unwilling to take on that responsibility for many reasons - some of which may seem frivolous to others - like, the child is disabled...
One could argue that then those sort of people should not do the things that make babies and so we just push back to whether contraception itself is a crime against humanity because of all the ova that don't get fertilized and thus rob potential people of ever getting the chance to be people - and the very fact that men produce orders of magnitude more sperm cells than there are ova means that trillions of sperm are denied their right to become people.
The universe really doesn't care about our morality - it is utterly oblivious and indifferent to us so we have still births and sudden infant death and all sorts of plagues of sudden and uninvited deaths. We can draw our arbitrary lines and each can convince themselves that their lines are the right ones but reality will always confound us. We just look for reasonable compromises which are generally based on the distribution of power. People will generally try to maximize their evolutionary outcome by having the children they can support well rather than too many and that depends on their environment whereas slave owners would rather maximize their herd and maximize the necessity for the herd to slave.