The admirable American debate occurs within some non-debatable parameters, beginning with this: Human life begins at conception, a conclusion not of abstruse philosophy or theology but of elementary biology. But this is not, as many abortion opponents think, where the debate about abortion ends. Rather, for most Americans it begins here: When is it reasonable — in some sense objective, because visible — to see a human person?

This is why technological development has done much to stabilize the politics of abortion by enlarging and solidifying the ambivalent majority in the middle. Vastly improved sonograms present vivid pictures of small persons in utero, with beating hearts, and sucking their thumbs. Persons who, in the formulation of one antiabortion activist, can hear their mothers’ heartbeats.

Americans who believe in a “right to life” are right that, absent a mishap or an abortion, the life that begins at conception becomes, in utero, recognizably a person. But when?