A 20-year-old primigravida attends her first antenatal visit. Transabdominal ultrasound shows a single intrauterine pregnancy with a crown-rump length (CRL) of 65 mm. However, the patient is unsure of her last menstrual period (LMP), and her menstrual cycles are irregular (28–35 days). The sonographer notes mild ventriculomegaly (atrial diameter 10 mm) and a single umbilical artery. What is the most appropriate next investigation to confirm gestational age and assess the significance of these findings?