A CTG machine uses 2 monitor leads from the machine, to be strapped to the woman's belly. The end of one lead uses ultrasound or
Doppler to continuously record the baby's heart rate (or the amount of beats per minute or 'bpm'). This has conducting gel applied to it (similar to the type used when you have an ultrasound) and is placed on the woman's belly around about where the caregiver thinks the baby's shoulder is lying inside the uterus. The closer the lead is to the baby's shoulder, the more accurate, or clear, the readout will be.
The other lead is attached to a small 'transducer' and is placed on the woman's
fundus. The transducer detects any contractions the woman may be experiencing (whether they be painful labour contractions, or painless Braxton Hicks contractions). Placing both leads on the outside of the woman's belly is referred to as 'external monitoring'.
Another form of continuous monitoring is called 'internal monitoring'. This refers to when the ultrasound lead is replaced with a wire and clip, placed inside the woman's vagina and attached to the baby's head. The waters need to be broken to perform internal monitoring.
Image 3-17 shows a woman being monitored with an external CTG Machine. The monitor on the top of her belly picks up any Braxton Hicks or labour contractions. The lower monitor detects the baby's heart rate continuously.
Both leads produce a visual, digital number readout on the machine, as well as a printed record onto a continuous strip of paper. Your caregiver reads and interprets the pattern of your baby's heart rate, to help them make a judgment about the well-being of your baby, and possibly if any interventions are needed.
Image 3-33 shows one CTG Machine, with the continuous recording of the baby's heart rate and contractions.
The 3 main patterns on the printed readout can be:
The 'variability' of the heart rate.