The existence of a thing does not depend on the number of people who believe in it. And not everything that exists has legal standing. So far as I know, there is no legal definition of a soul in the U.S., (or maybe any Western country) nor are there any crimes or torts involving or pertaining to a soul.
For legal purposes, the standard which pertains to personhood has to do with the potential for brain activity as an indicator of prospects for a self-meaningful existence as a person, and where human organisms are concerned, we tend to have an expansive view of what could constitute a self-meaningful existence. (We don’t extend this consideration to other species, because we have rigged the definition of person so that only human organisms can qualify.) We define the termination of personhood when brain activity irrevocably drops below a level that we think could support sensation or thought. If the lack of brain activity is not irrevocable–as in cases of severe hypothermia, for example–we continue the status of personhood based on the potential for future brain activity. A fetus in fetu might have brain activity, but it would still not qualify as a person on the more fundamental grounds of having no prospects of a meaningful existence.
It seems fairly straightforward that a single cell or clump of cells is not a person. It has no potential for a self-meaningful existence. There are some who try to stretch the “potential” argument by saying it would eventually reach a stage of brain activity, but what they are really saying is that the clump of cells has the potential to develop into a very different organism, and it is that organism which would acquire the potential for brain activity. They are, in effect, trying to shoehorn in a “potential for potential” argument. The clump of cells would never have the potential for brain activity as it is. And if you open the door to potential for potential, then obviously individual spermatozoa and eggs have that as well.
But at the other end of pregnancy, the commonsense view is that a viable fetus with brain activity and the ability to respond to stimuli has reached the point of potential for meaningful existence, even while it retains an umbilicus. (All indications are that consciousness precedes birth–even to the point of having in utero wake and sleep cycles.) The carrier is still deserving of primary consideration so long as she or he is still a person, because actual personhood takes precedence over potential personhood. (A permanently brain-dead carrier with a living fetus loses primacy, and becomes, in effect, a piece of incubation equipment.)
Also, to be clear, feticide is simply the termination of a fetus. Whether it counts as criminal or legal depends on the circumstances of the case. But even criminal feticide is generally held to be something distinct from murder.