I can give you a point of view connected to history of Law, as, basically, when someone is considered to be "alive" for civil obligation general purposes (being part on a contract even if represented, own property even if supervised, have the right to inheritance, be entitled to basic human rights, and so on).

Actually, under that point of view there are two historical perspectives:

- a first, more ethical/religious perspective, that considers "a soul" and life as connected to the existence or incarnation of that soul, and that resides more in the field of moral considerations (so, foreign to social and law science and the considerations a lawmaker must keep in mind while formulating statutes of any kind, as Law, as science, shouldn't, ideally, deal with moral and ethical variations, but be constructed as an instrument to preserve material and immaterial concrete assets (such as life, property, etc) as those assets are deemed as valuable enough by the majority of society)

- a second, strictly connected to the science of Law, as the instrument to create general abiding rules for a society to be governed by, through the State (or whatever/whoever holds the monopoly of coercive power through the ability to enact legally authorized violence, which is basically the definition of State and a Legal system). Under that second perspective, a fetus won't generally be considered a person until medical science becomes capable, through scientific techniques, to determine if that fetus is a sentient being, meaning an independent person with its own thoughts and feelings.

As result of that legal principle, all Civil Law jurisdictions, and some Common Law jurisdictions as well, only considered someone to "be alive" after they "were born alive", meaning that they were taken out of their mother's womb, and breathed air, even if briefly.

To legally determine that status, a scientific test was designed, based on Galen's findings regarding the differences between the lungs of stillborn and the lungs of baby that was born alive, as he demonstrated in his work "De usu partium". That test is called Docimasia Pulmonum Hydrostatica of Galen, and consists, basically, on extracting the lungs of a dead baby that was suspected to be stillborn and to throw it in a water container. If the baby had breathed air, and so was LEGALLY ALIVE, even if for a brief moment, the lungs will float. If the baby was a stillborn, and as so was not LEGALLY ALIVE, the lungs will sink to the bottom.

So, anti-Abortion laws had not in mind the "rights of the unborn baby" but the rights of the mother, as the unborn baby was considered as PART OF THE MOTHER'S BODY, and/or the PROPERTY RIGHT of the father, as the unborn baby, as part of the mother, was also property of the father, as women were considered to be property of their husbands as they were before their fathers' property until marriage.

During the 17th century, UK Law began to consider the docimasia hydrostatica as a test to determine if infanticide was committed in case of a baby's death during delivery. Mother, or whoever assisted during delivery, were only considered suspects of infanticide if the baby was born alive as proved by the docimasia results. If the baby was stillborn, he wasn't considered as a person, and as such couldn't be the victim of a crime.

In some Common Law jurisdictions, especially here in the US, and especially on those regions more influenced by religious and anti-science movements in general, the moral aspect of the status regarding the life of a unborn baby began to take precedence over the traditional point of view of legal science, and anti-abortion statutes (laws and court rulings) began to criminalize abortion based on the moral argument of the value of the life of the unborn baby, and its soul, suffering, pain, and so on, by the end of the 19th century.

Only with the advances of medical science in the 1960s and 1970s is that a scientific "proof of life" of a unborn baby became, somehow, viable. Still, to prove that unborn babies are sentient beings, beyond shadow of doubt, as Law demands, is something that still eludes medical science, and, as result, most Civil Law jurisdictions still consider that LEGALLY, someone can only be considered alive (and entitled to all unconditional rights a human being shall be entitled to) if he or she was born alive, as, again, by breathing air independently, even if for a brief moment.

This is very important as far as inheritance and succession rights go, because, as for example in some jurisdictions, if a woman was not legally entitled to inherit property from a man, by having a son or a daughter fathered by that man that woman could become entitled to receive part of that man inheritance, as mother of his son or daughter, even that son or daughter died immediately after being born, while if that baby was stillborn, she won't have any right to a claim over that man's estate.

A brief text about the history of docimasia can be found here, with lots of good references.