Ultrasound representative images of the foetal aorta. (A): Aortic arch showing the position of the aortic isthmus (I) and the proximal descending aorta (*). (B), (C) and (D): Doppler flow waveforms at the first (B), second (C) and third (D) trimesters, respectively. The flow profile in the foetal aorta is characterized by a steep systolic rise, a postsystolic notch and a low end-diastolic forward flow velocities. The acceleration time is a reflection of cardiac contractility, while the subsequent diastolic phase reflects peripheral vascular impedance. The postsystolic notch develops as a result of aortic valve closure during early ventricular diastole resulting in a slight decrease in flow velocity.