Three sisters were arrested in the death of a baby found in a garbage bag on a back porch, police said. Laura Sergio, 25, who is believed to be the mother, was arrested Saturday on a murder charge, police said. Her sister Andria, 27, was arrested on a charge of improper disposal of a human body. Another sister, Cara, 29, was arrested on a charge of obstructing governmental administration. Laura Sergio was taken to a hospital with a medical problem where a doctor called authorities to say she might have given birth, according to police. They were directed to a Brooklyn home, where they found the infant unconscious around 3:30 a.m. Saturday. The baby was pronounced dead at a local hospital, police said. Medical examiners later determined the infant had been carried to full term and born alive, said a spokeswoman for the city medical examiner’s office. The baby died of asphyxiation and hypothermia, she said.