Cardiotocography (CTG)

Cardiotocography (CTG)

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Cardiotocography (CTG) is a technical means of recording (-graphy) the fetal heartbeat (cardio-) and the uterine contractions (-toco-) during pregnancy, typically in the third trimester. The machine used to perform the monitoring is called a cardiotocograph, more commonly known as an electronic fetal monitor. CTG can be used to identify signs of fetal distress.


Simultaneous recordings are performed by two separate transducers, one for the measurement of the fetal heart rate and a second one for the uterine contractions. Transducers may be either external or internal.

External measurement means strapping the two transducers to the abdominal wall

  • The pressure-sensitive contraction transducer, called a tocodynamometer (toco), measures the tension of the maternal abdominal wall – an indirect measure of the intrauterine pressure
  • The fetal heart rate transducer overlays the fetal heart, measures the fetal heart rate

Internal monitoring differs from external monitoring.

  • The pressure-sensitive contraction transducer, called a tocodynamometer (toco), measures the tension of the maternal abdominal wall – an indirect measure of the intrauterine pressure
  • The fetal heart rate transducer is replaced by a smaller lead that is placed inside the woman’s vagina and attached to the head of the baby. The internal lead is called a ‘fetal scalp electrode’ (or FSE). It is ONLY used to monitor the baby’s heart rate during labour, usually if external monitoring is not being reliable.

A fetal scalp electrode is a small, circular, corkscrew-shaped needle attached to a coated wire. The clip is covered with a long, protective, flexible, plastic covering and guided up through the mother’s vagina by the caregiver doing an internal examination. The needle is gently rotated into the SKIN on the baby’s scalp. Once the clip is attached, the plastic cover is removed, leaving just the wire.

The fetal scalp clip has 2 coloured wires attached. The wires are connected to the lead with a small conducting device (about the size of a match-box), strapped to the woman’s thigh. The lead is then plugged into the monitor and a typical CTG reading is printed on paper and/or stored on a computer for later reference. Use of CTG and a computer network allows continual remote surveillance: a single nurse, midwife, or physician can watch the CTG traces of multiple patients simultaneously, via a computer station.

This procedure should not be any more uncomfortable than a normal vaginal examination.

The internal electrode monitors the baby’s heart rate more accurately than an external Doppler.

Internal measurement requires a certain degree of cervical dilatation and the waters need to be broken to attach an FSE to the baby’s head. If they are not already broken, this will need to be done to allow the electrode to be attached.

Women who carry the Herpes, Hepatitis B or C or HIV viruses are recommended not to have internal monitoring, because it can increase the baby’s chances of becoming infected with these viruses.


Cardiotocography is used to monitor several different measures:

  • Uterine contractions
  • Four fetal heart rate features
    • Baseline heart rate
    • Variability
    • Accelerations
    • Decelerations

A typical CTG output for a woman not in labour. A: Fetal heartbeat; B: Indicator showing movements felt by mother (caused by pressing a button); C: Fetal movement; D: Uterine contractions

Uterine contractions – They are quantified as the number of contractions present in a 10 min period and averaged over 30 min.

  • Normal: ≤ 5 contractions in 10 min.
  • High: ≥  5 contractions in 10 min represent uterine tachysystole

Baseline heart rate – average baseline fetal heart rate

  • Reassuring feature: 110 – 160 beat per minute (bpm)
  • Non-reassuring feature: 100 – 109 bpm  OR  161 – 180 bpm
  • Abnormal feature: < 100 bpm  OR  > 180 bpm

Variability – Fluctuations in the fetal heart rate
This causes the tracing to appear as a jagged, rather than a smooth, line. Variability is indicative of a mature fetal neurologic system and is seen as a measure of fetal reserve

  • Reassuring feature: ≥ 5 bpm
  • Non reassuring feature: < 5 bpm for ≥ 40 minutes but < 90 minutes
  • Abnormal feature: < 5 bpm for > 90 minutes

Decreased variability may occur in the following situations:

  • Hypoxia and Acidosis. The lack of oxygen and the build-up of acid in the fetal body depress the fetal heart and nervous system
  • Narcotics and anaesthetic agents, depress the fetal nervous system.  Usually, variability increases as the drug is eliminated from the baby.
  • Prematurity.  The fetal nervous system in a premature baby cannot effectively control the heart rate.
  • Fetal sleep (as noted above).


Increases in fetal heart rate from the baseline by at least 15 beats per minute, lasting for at least 15 seconds and should be 2 accelerations every 20 minutes lasting no longer than 2 minutes.

  • Reassuring feature: Present
  • In labour, the absence of accelerations with an otherwise normal CTG is of uncertain significance


Decreases in fetal heart rate from the baseline by at least 15 beats per minute, lasting for at least 15 seconds

There are three types of decelerations, depending on their relationship with uterine contraction:

  1. Early deceleration
    1. Begin at start of uterine contraction and end with conclusion of contraction (mirror image)
    2. Due to increased vagal tone due to fetal head compression
  2. Variable deceleration
    1. Occur at any time irrespective of uterine contractions
    2. Due to umbilical cord compression
  3. Late deceleration

    1. Begin at or after the peak of a contraction and ends long after it, hence the “late” when compared to early decelerations
    2. Due to fetal hypoxia (uteroplacental insufficiency) – the most worrisome deceleration
  • Reassuring feature: No deceleration
  • Non reassuring feature: Early deceleration, variable deceleration or single prolonged deceleration up to 3 minutes
  • Abnormal feature: Atypical variable decelerations, late deceleration or single prolonged deceleration greater than 3 minutes


  • Normal trace: Tracings with all four features:
    • Baseline rate 110-160 bpm,
    • Normal variability,
    • Absence of decelerations,
    • Accelerations (may or may not be present)
  • Suspicious trace: 
    • Tracing with ONE non reassuring feature and the other three are reassuring.
    • This is not predictive of abnormal fetal acid-base status, but evaluation and continued surveillance and reevaluations is indicated
  • Pathological trace:
    • Tracing with TWO or more non reassuring features or ONE or more abnormal feature
    • It predicts abnormal fetal acid-base status; this requires prompt evaluation and management

Reasons to monitor the baby’s heart rate can include:

  • If there are health concerns for the mother during late pregnancy
    • Bleeding
    • High blood pressure
    • Premature labour
    • Diabetes
    • Cholestasis
  • If there are health concerns for the baby
    • Small for dates
    • Abnormality of the baby
    • Pregnancy is overdue
  • As a routine procedure during labour
    • Some delivery suites have a policy to perform a routine 20 minute recording of the baby’s heart rate when the woman arrives in labour
  • If the waters break and the labour has not started
    • If the woman is waiting for her labour to start with ruptured membranes, the caregiver may perform a 20 minute trace to check her baby’s well being.
  • If the baby is showing signs of distress
    • If the baby’s heart rate is noticed to be lower, when the caregiver is listening intermittently (usually after a contraction during labour)
    • If the waters break and there is meconium staining of the amniotic fluid.
  • If the labour is being induced or augmented with an oxytocin drip or prostaglandins.
    • These medications have the potential to overstimulate the uterus and distress the baby.
  • If the woman is having an epidural for pain relief in labour.
    • The baby’s heart rate can lower in response to a fall in the woman’s blood pressure, a possible side effect of an epidural.

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