Analysis of the FHR

The study of the fetal heart rate is carried out using a cardiotocograph (CTG monitor) (Fig. 1.2), which records the heart rate by means of a Doppler ultrasound transducer placed, after locating it by ear, over the fetal cardiac area. The sampling frequency is 4 Hz, i.e. 1 signal every 250 ms, following a calculation by autocorrelation (Fig. 1.3).

The uterine contraction (UC) transducer is placed on the uterine fundus; this also picks up the maternal heart rate. Placed on the uterine fundus, the transducer only reflects variations in tension at the level of the abdominal wall (including the uterus and abdominal muscles). This technique therefore does not measure intrauterine pressure. The basal tone is artificially set on the monitor at 10 or 20 mmHg and must be redefined according to changes in the patient's position.