As discussed in a previous post, the theory of acquired rights asserts that a child qualifies as a rights-bearing person once he or she demonstrates possession of some characteristic or ability that confers rights-worthiness.
One version of this theory argues that when a child has the potential to be capable of reasoning at some point in the future, he qualifies for rights. Whatever this argument's merits, it does not support the theory of acquired rights. If rights are based on a child's future capacity to reason, then rights are inherent, not acquired.
If rights come from the potential to reason in future and not from the ability to reason now, then the current characteristics and abilities of the child are irrelevant. It does not matter what a child looks like or can do at present, since this argument for rights is based entirely on something that a child could potentially do in future. The logical implication of this argument is that a child has rights from the moment he comes into existence, since from that moment onwards there is the potential of a future in which the child is capable of reasoning.
Advocates of the theory of acquired rights refuse to accept this logical implication. They attempt to argue that an unborn child only has rights once it reaches certain developmental milestones that are related to a future ability to reason, but not before it has reached those milestones. For example, Jason Sorens argues that an unborn child only acquires rights at six and a half months because at that point the child has various features including "a recognizably human brain" which gives it the "potential for rationality".
However, any arbitrary milestone is completely irrelevant to the basic premise of the argument. As a reminder, the argument is that a child has rights despite his current inability to reason. Rather, he has them because he has the potential to reason in future. The logical implication is that rights are valid from the moment of conception, not from any point in brain development, since from the moment of conception a child is unable to reason but has the potential to reason in future. There is no philosophical basis for arguing that a child acquires rights because of any developmental milestone if that milestone isn't actually changing the child from a non-reasoner to a reasoner.
Advocates of this version of the theory of acquired rights are trying to argue that the potential for reasoning in future is a valid criterion for rights, but it should only be taken into account once some arbitrary developmental milestone has been reached by a child. The entire endeavour is nonsensical. It appears to be motivated reasoning with a particular conclusion in mind: to set aside a window of time in which a child does not have rights so that abortion is not considered a rights violation.