A 32-year-old primigravida presents to the antenatal clinic at 12 weeks of gestation (by LMP). She is uncertain about her menstrual cycle regularity and reports that her last menstrual period was "approximately 3 months ago." Transabdominal ultrasound shows a crown-rump length (CRL) of 48 mm. The patient is concerned about the discrepancy between her stated dates and the ultrasound findings. What is the most appropriate next step in dating this pregnancy?