For example, could a fetus-person donate to a political campaign? Vote? Open a bank account? And would a fetus-person have the same legal rights as a corporate person, namely, to enter into and enforce contracts? Children don't have these rights, but the Personhood Amendments don't define fetuses as children. Hmmm. I guess all the personhood-states are going to have to tinker with the definition of a fetus-person to differentiate it from other kinds of persons. But, if you take away some rights from fetus-persons, what other rights could they lose?
The case of children is illustrative. Western society started, back in Roman times, with the notion of children as property of their fathers or guardians. There wasn't much change until the 19th century, when the first real rights were granted to children as individuals separate from their parents in the forms of compulsory education and child labor laws, innovations that conservatives, then and now, decry as unnecessary intrusions of the state into familial life. Since then, there have been numerous debates and developments in children's rights, in the US most notably over health and welfare issues. To create a new category of fetus-persons, protected from their parents by the state would open up a similarly complex set of issues that would not be resolved quickly or easily, and that would undermine fundamental conservative principles.
If the state has a declared interest in the well-being of the fetus-person, can pregnant women smoke? Drink? Can the neighbors play loud music? Fetuses are far more vulnerable to toxins, which greatly increase the risk of miscarriage; would fetus-personhood create a higher burden on industry to maintain environmental standards? On food manufacturers to limit additives in foods consumed by pregnant women? All of these questions would be raised, and probably litigated. Have the supporters of these measures considered these
Then there's the issue of taxes. The fetus-person would, arguably, be a resident. If the personhood-states determine that they are, somehow, legal, then under the US tax code, they could be claimed as a dependents by their mothers. Every expectant mom in a personhood-state gets a tax deduction! Even with the current makeup of the bench, I can't see the Supreme Court allowing individual states to define an entirely new category of federal tax deductions. At the very least, the definition of personhood would have to be modified to 'personhood for the purposes of' something or other, which would fundamentally undermine the premise that the fetal-person's rights should ever be considered equal to those of a full citizen.
Which brings up the question of fetus-person citizenship. We can answer that one. In the US, one of the ways we define persons is by whether or not they are US citizens. We grant citizenship in three ways:
1) By birth to a parent who is a US citizen,
2) by birth on US soil, or
3) by naturalization.
A fetus-person, not having been born, would be ineligible for methods one and two, and thus not inherently a citizen.
This has some interesting implications. Every pregnant woman in a personhood-state would be harboring an illegal alien and subject to prosecution. To avoid this without amending the US Constitution, all those fetuses would have to be naturalized, a process they could, arguably, be eligible for as immediate relatives of US citizens. This process is, of course, the responsibility of US Citizenship and Immigration Services, a federal agency. In 2010, around 675,000 applications for naturalization were processed, but it jumps around a lot - the highest number in the last decade was over a million in 2008. There are about six million pregnancies in the US each year, with just under two million of those ending in miscarriages. Even leaving out the miscarriages, that would, at the very least, quintuple the workload for naturalization services, requiring a significant expansion in the workforce. That being the federal workforce, you understand. Not to mention the trouble the states would have dealing with what to do with their growing illegal-fetus-aliens during the six months it takes USCIS to process each of these requests.
Far more likely is that the federal government would decide that USCIS will not accept naturalization requests for pre-born future-citizens. In that case, personhood-states would simply have to revise their immigration laws to accommodate large populations of temporary, illegal-fetus-aliens by defining them as some other category, like 'future citizens'. But, any fetus that can reasonably expect to be born on US soil is a future citizen. It would be very tricky to write that definition in a way that did not automatically grant temporary immunity from deportation for all pregnant women.