SAN FRANCISCO — A Northern California woman has been accused of knowingly killing her infant son by breast-feeding him shortly after ingesting large amounts of methamphetamine, which rendered her milk toxic.

The woman, Maggie Jean Wortmon, 26, was arraigned on Wednesday in Humboldt County Superior Court on charges of second-degree murder — punishable by 15 years to life — along with child abuse and involuntary manslaughter. The arraignment stems from the death of her 6-week-old son, Michael, who was found unresponsive one November morning last year in the town of Loleta, Calif., north of San Francisco.

The baby was pronounced dead at a local hospital, and was later discovered to have methamphetamine in his system. This year, Ms. Wortmon was charged with child endangerment and involuntary manslaughter.

But on Wednesday, a charge of murder was added after prosecutors said that they had determined that Ms. Wortmon had shown “conscious disregard” for her child by using drugs and then nursing.