29 states have fetal homicide laws, like the one Scott Peterson is charged with violating. Here's a chart of the laws, courtesy of the Alan Guttmacher Institute.

The Laci Peterson case in California has brought renewed attention to the question of the application of homicide statutes to a fetus. California penal law defines murder as “the unlawful killing of a human being, or a fetus, with malice aforethought.”

In a subsequent 1994 decision in People v Davis, the California Supreme Court held that a fetus that has “progressed beyond the embryonic state of seven to eight weeks” can be considered a victim under the statute.

California is one of 29 states in which at least one section of the state’s homicide statute includes a fetus as a victim at some point in gestation.... State homicide statutes often cover several different crimes, ranging from capital murder to manslaughter to vehicular homicide. The laws often require different levels of intent for these different crimes and additionally, may have different requirements concerning knowledge of pregnancy. Within most homicide statutes, however, the point in pregnancy at which a fetus is considered a victim does not vary from one crime to another.

[link via Paper Chase]