All the sonographic examinations were performed by the same operator (FM) at the same time of day (between 19.00 and 22.00 h), using an SSD-5500 ultrasound machine (Aloka) equipped with a 7.5 MHz linear transducer (UST-900L2-7.5, Aloka). Uterine blood flow was assessed using the blood flow volume (BFV, mL/min) and pulsatility index (PI).