A 28-year-old Indian woman with a history of classical phenylketonuria (PKU), well-controlled on a phenylalanine-restricted diet since infancy, presents to the antenatal clinic at 8 weeks of gestation. Her recent plasma phenylalanine level is 18 mg/dL (normal <2 mg/dL). Ultrasound shows a structurally normal fetus. Which of the following is the most appropriate next step in management?