Until I saw this post by Chris Brooks at Homeward Bound, I'd never encountered any pro-choice use of Numbers 5. The passage describes a procedure for determining whether a woman accused of adultery was guilty or innocent. It's generated a lot of discussion for other reasons, and since I read two commentaries on Numbers a few years ago I remember a lot of the issues that occur in those discussions. But abortion wasn't one of them, as far as I remember.
The pro-choice use of this passage is as follows. The penalty for a divine determination of guilt is for the woman's stomach to swell and for her to become barren. Pro-choicers then say that if she's already pregnant as a result of her adulterous relationship (which would happen often enough that it's going to matter for a lot of cases over many years) then the punishment would mean the death of the fetus. That reveals God's attitude toward fetuses that they don't have the kind of moral status adults have.
Now there are a number of things to say about this argument. Chris said some of them. But one thing in particular makes me think of this argument as completely crazy, and it didn't occur to me until I saw commenter Vinny's response to a comment I had left. Suppose following this procedure would lead God to cause a miscarriage every time the woman was pregnant and really had been unfaithful. Why couldn't God just prevent conception in the cases where he knew he was going to judge someone in this way? Vinny is assuming God couldn't.
But that kind of response is even unnecessary. Think about all the people God causes to die throughout the pages of the Bible. Some of them are punished for outright sins, such as Uzzah's refusal to follow the prescribed manner of carrying the ark when he touches it, Ananias and Sapphira's willingness to lie about how much they'd given to appear to have given everything they'd gotten, or Aaron's two oldest sons' burning of strange fire in the early days of the tabernacle, contrary to God's command only to burn a certain mixture of incense with a specific recipe. On the other hand, some people die because of other people's sins, and sometimes this is directly decreed by God. David and Bathsheba's first child dies as a judgment for their sin, a nice parallel of an infant in the same circumstance as Numbers 5 would be describing for a fetus if it indeed implies a miscarriage. God's judgment for David's census involves a very large number of people dying, and the same is true of a few occasions during the wilderness wanderings in Exodus and Numbers, where likely not everyone who died was guilty.
So it may well be that Numbers 5 reveals God's attitude toward the unborn. I'll grant that as long as the pro-choicer grants that these other passages reveal God's attitude toward adult human beings, even ones innocent with regard to the crime being punished. Once that's clear, it's very hard to make this pro-choice argument without also claiming that it's ok to kill adult human beings because God does so. Murder is still forbidden, even if there cases where God kills a human being in judgment for someone else's sin. You can't infer a lower moral status of a fetus from Numbers 5, because you'd also have to infer the same lower moral status for adults based on other biblical passages.
True story: By Jewish law, a baby/fetus was not considered a person unless it was 30 days of age, AFTER birth. Therefore it's fallacious to compare an action to an adult human being with an action to a fetus.
By U.S. law, slaves were counted as 1/4 of a person for the sake of determining representation of each state in Congress, but they were counted as not persons for the sake of voting to determine who would be representing them. By your logic (i.e. that if something is treated a certain way by law then it is that way) it was actually true that they weren't fully persons but were partial persons, and it was also true that they were not at all persons.
That assumes that your claim is even true. I actually have a pretty hard time believing Jewish law (and I'm not sure what time period or set of laws you're referring to) declared what it is to be a person. If you mean the Torah, it's certainly not true. The Torah never defines personhood. If you mean later Jewish law, it's irrelevant (for the reasons in the above paragraph) but also extremely unlikely. The concept of personhood was first presented as a way of sorting out the seemingly-contradictory claims in Christian theology involving the Trinity. I very much doubt that any Jewish laws in the pre-modern world would have concerned themselves with such a concept. Even today, with contemporary philosophy making the idea of personhood relevant in an artificial way, you don't have people passing laws to justify abortion by defining personhood (although there's now been one attempt to pass a law defining a fetus as a person in order to move the law the other direction). So I'm as bit skeptical of your claim even if it were relevant.