A bad moral argument about the humanity of the unborn in support of a good law restricting abortion

One of the most well-argued books I've recently read on the topic of abortion is Francis J. Beckwith, Defending Life: A Moral and Legal Case Against Abortion Choice (Cambridge University Press 2007). Because of a project I'm working on this summer, my attention was drawn in particular to Beckwith's argument against resting the humanity of the unborn on human appearance after a certain point of development. (A version of Beckwith's arguments, which draw on John Jefferson Davis, Abortion and the Christian: What Every Believer Should Know, is available here.) 

Beckwith is right to observe that the human body takes on a variety of forms over the course of life. An elderly person does not have the body of a teenager, a teenager does not have the body of an infant, nor does an infant have the body of an embryo. A healthy, developing embryo at a particular time looks just like a healthy, developing embryo is supposed to look at that time. In Beckwith's words, "the unborn at any stage of her development looks perfectly human because that is what humans look like at that time." We risk confusing appearance with reality if we rest human moral worth on a certain type of human appearance. 

But let us not be too hasty in pushing aside the moral significance of a baby's obviously human appearance. In designing laws, it can be helpful to meet people where they're at. Consider the possible legal significance of the first three search results that popped up just now when I googled "pregnancy at twelve weeks":

A baby at twelve weeks gestational age has an obviously human body. If a baby has an obviously human body, isn't it reasonable for us as a people and for our government as a government to treat that baby as a human baby? And don't human babies deserve the equal protection of the laws?

I haven't mentioned anything yet about human personhood. For the moment, though, let's stick with the basic point that a baby with an obviously human body is obviously a human baby. Let's add in a couple other characteristics to the obviously human body, such as life and healthy normal development. Should the law truly be powerless to protect this human being? 

It's around this point that people interject considerations that tease apart the categories of human baby and human person. In unselfconscious reversal of the normal charge that pro-lifers are trying to impose their religious views about human personhood, those who seek to deny the human moral worth of babies with an obviously human body tend to rely on a controversial metaphysical claim. There is some property or quality, these people argue, that a human being with an obviously human body must _also_ possess in order to be a human person. But why should we let a controversial metaphysical position of this sort displace the idea that human bodiliness--whatever its relation to "full humanity" might be--is enough to bring a human being within the protective reach of positive law?