Review Question - QID 109256

A 36-year-old G3P2002 woman presents to her obstetrician’s office for her 1st prenatal visit at 10 weeks and 2 days gestation. She has felt nauseous the last several mornings and has been tired for a few weeks. The patient has had 2 uncomplicated spontaneous vaginal deliveries at full term with her last child born 6 years ago. She is concerned about the risk of Down syndrome in this fetus, as her sister gave birth to an affected child at age 43. She has a history of generalized anxiety disorder, atopic dermatitis, and she is currently on escitalopram. Her temperature is 98.6°F (37.0°C), pulse is 70/min, blood pressure is 121/67 mmHg, and respirations are 13/min. The patient appears anxious, but overall comfortable, and cardiopulmonary and abdominal exams are unremarkable. Pelvic exam reveals normal external genitalia, a closed and soft cervix, a 10-week-sized uterus, and no adnexal masses. Which of the following is the most appropriate next step for definitively determining whether this patient’s fetus has Down syndrome?