Fetal Monitor

Overview

A fetal monitor records two signals side by side during labor: the fetal heart rate and the timing and strength of the mother's uterine contractions. Plotted together on a moving strip, this cardiotocograph lets clinicians judge how the fetus is tolerating labor, a heart-rate dip that follows each contraction means something different from one that precedes it, and the shape of those patterns guides whether labor continues or the team intervenes.

The fetal heartbeat is picked up by the Doppler Ultrasound Transducer, a Doppler ultrasound probe strapped to the abdomen, and contractions by the Toco Transducer, a pressure button that senses the belly tensing. Both feed the Main Board, which extracts heart rate and contraction strength and drives the Display and the Thermal Printer that prints the permanent CTG strip. A Speaker plays the audible heartbeat, a Battery Pack and medical Power Supply run the unit, and the Connector Panel takes the color-coded leads.

How it works

The Doppler Ultrasound Transducer holds a ring of Piezoelectric Element crystals that emit ultrasound pulses and listen for the echoes. Moving heart walls and valves shift the echo frequency, the Doppler effect, and the Main Board measures that shift to recover the beating motion. Because the raw Doppler signal is noisy and the fetal heart is a small moving target, the board autocorrelates successive beats to lock onto a stable rate rather than counting individual edges, which is why the displayed number is smoothed and updated each beat. The same audio drives the Speaker so the midwife hears the characteristic galloping heartbeat.

Contractions are sensed mechanically. The Toco Transducer presses a spring-loaded guard button against the abdomen; as the uterus contracts the wall stiffens and pushes the button in, and a Pressure Sensor turns that deflection into the TOCO trace. It reads relative tension, not true intramuscular pressure, so it shows contraction timing and frequency rather than an absolute value. The Thermal Printer commits everything to paper: a Paper Feed Stepper advances the chart at a fixed speed while a thermal printhead burns the heart-rate and contraction curves onto heat-sensitive gridded paper, giving a continuous legal record that does not depend on the monitor staying powered.