A 38-year-old G4P3003 woman presents for a follow-up appointment at 12 weeks and 5 days gestation. She had undergone non-invasive prenatal testing (NIPT) at 10 weeks, which indicated a high probability for Trisomy 21. She reports feeling increasingly anxious about the results and is eager to discuss options for definitive diagnosis. Her medical history includes well-controlled generalized anxiety disorder, for which she continues to take escitalopram, and a history of atopic dermatitis. Her previous pregnancies were uncomplicated, resulting in three healthy full-term vaginal deliveries. She also mentions her elder sister, who had a child with Down syndrome at age 43, contributing to her heightened concern. On examination, her vital signs are stable: temperature 37.0°C, pulse 72/min, blood pressure 125/70 mmHg, and respirations 14/min. Physical examination, including a focused abdominal and pelvic assessment, is unremarkable, consistent with her gestational age. Given her NIPT results, advanced maternal age, and family history, which definitive diagnostic procedure is most appropriate at this gestational age, and what initial counseling should be provided if a trisomy 21 diagnosis is confirmed?