10.1 Intrapartum Fetal Heart Rate Monitoring & CTG Interpretation (NICE / FIGO)

Intrapartum Fetal Heart Rate Monitoring & CTG Interpretation (NICE / FIGO)

Intrapartum fetal surveillance is a fundamental core competency in midwifery practice, designed to assess fetal oxygenation throughout labour and identify fetal compromise before irreversible hypoxic-ischaemic neurological injury or intrapartum fetal demise occurs. In the United Kingdom, intrapartum fetal heart rate monitoring is guided by the National Institute for Health and Care Excellence (NICE) guideline NG229 (Fetal monitoring in labour) and the International Federation of Gynecology and Obstetrics (FIGO) consensus guidelines on intrapartum fetal monitoring. Midwives must synthesize maternal clinical context, labour progress, and cardiotocograph (CTG) features to differentiate physiological fetal adaptation from progressive acidaemia.

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Physiological Control of the Fetal Heart Rate

The fetal heart rate (FHR) is continuously regulated by an intricate homeostatic balance between the central nervous system, autonomic pathways, and vascular receptors:

1. Autonomic Nervous System Regulation

• Parasympathetic Nervous System: Mediated primarily by the vagus nerve (Cranial Nerve X), which innervates the sinoatrial (SA) and atrioventricular (AV) nodes. Vagal stimulation releases acetylcholine, which slows the heart rate and produces rapid, beat-to-beat oscillatory changes. This parasympathetic influence matures progressively with gestational age and is the primary driver of normal baseline variability.
• Sympathetic Nervous System: Mediated via cervical and thoracic sympathetic chain ganglia releasing norepinephrine and circulating catecholamines from the fetal adrenal medulla. Sympathetic tone increases the baseline heart rate, enhances myocardial contractility, and elevates cardiac output. Sympathetic activity predominates during somatic fetal movements, generating transient accelerations.

2. Receptor-Mediated Reflexes

• Baroreceptors: Located within the aortic arch and carotid sinuses. These stretch-sensitive receptors respond instantaneously to sudden elevations in fetal arterial blood pressure (such as occurs during mechanical compression of the umbilical cord) by triggering a rapid vagal efferent discharge. This baroreceptor reflex produces the abrupt, sharp-onset decelerations characteristic of variable decelerations.
• Chemoreceptors:
  • Peripheral chemoreceptors situated in the carotid and aortic bodies respond primarily to hypoxaemia (reduced partial pressure of arterial oxygen, PaO2) and hypercapnia (elevated PaCO2).
  • Central chemoreceptors in the medulla oblongata respond to severe hypercapnia and systemic acidaemia (reduced cerebrospinal fluid and blood pH).
  • Chemoreceptor stimulation triggers intense peripheral vasoconstriction (diverting oxygenated blood to the brain, myocardium, and adrenal glands) alongside vagal bradycardia, manifesting clinically as late decelerations.

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Intermittent Auscultation vs. Continuous Cardiotocography (CTG)

National guidelines delineate clear clinical criteria governing when intermittent auscultation is safe and when continuous electronic fetal monitoring must be instituted.

                    ┌──────────────────────────────────────────────┐
                    │      INTRAPARTUM SURVEILLANCE MODALITY       │
                    └──────────────────────┬───────────────────────┘
                                           │
         ┌─────────────────────────────────┴─────────────────────────────────┐
         ▼                                                                   ▼
┌──────────────────────────────────┐               ┌──────────────────────────────────┐
│     INTERMITTENT AUSCULTATION    │               │    CONTINUOUS ELECTRONIC CTG     │
│ • Low-risk women in labour       │               │ • High-risk antenatal conditions │
│ • Pinard or Doppler ultrasound   │               │ • Intrapartum risk conversions   │
│ • 1st stage: every 15 minutes    │               │ • Full 5-feature evaluation      │
│ • 2nd stage: every 5 minutes     │               │ • Continuous recording of FHR    │
│ • Auscultate for FULL 60 SECONDS │               │   and tocodynamometry            │
│   immediately post-contraction   │               │                                  │
└──────────────────────────────────┘               └──────────────────────────────────┘

Intermittent Auscultation (IA) Protocol

For healthy women with uncomplicated singleton pregnancies at term (37+0 to 41+6 weeks) in spontaneous labour, intermittent auscultation (IA) using a Pinard fetal stethoscope or Doppler ultrasound is the gold standard:

• First Stage of Labour: Auscultate the fetal heart rate for a full 60 seconds immediately following a uterine contraction at least every 15 minutes.
• Active Second Stage of Labour: Auscultate for a full 60 seconds immediately following a contraction at least every 5 minutes.
• Simultaneous Maternal Pulse: Palpate the maternal radial pulse simultaneously during every auscultation to ensure differentiation between maternal heart rate and fetal heart rate.

Indications for Converting from IA to Continuous CTG

NICE NG229 advises offering continuous CTG, and explaining why, if any of the following develop during labour:

• Maternal Factors: pyrexia (a single temperature of 38.0°C or above, or 37.5°C or above on 2 occasions 1 hour apart); suspected chorioamnionitis or sepsis; maternal pulse over 120 bpm on 2 occasions 30 minutes apart; severe hypertension (a single reading of 160 mmHg systolic or 110 mmHg diastolic or more); hypertension of 140/90 mmHg or more on 2 consecutive readings 30 minutes apart; 2+ proteinuria with a single raised blood pressure; pain that differs from normal contraction pain; fresh vaginal bleeding or blood-stained liquor.
• Fetal Factors: a possible fetal heart rate abnormality on intermittent auscultation (a baseline below 110 or above 160 bpm, a rising baseline, or decelerations), confirmed by more frequent auscultation; meconium-stained liquor.
• Labour Factors: contractions lasting more than 2 minutes, or 5 or more contractions in 10 minutes; confirmed delay in the first or second stage; use of oxytocin; insertion of regional analgesia.

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Systematic Feature-by-Feature CTG Evaluation

Interpretation of a continuous CTG requires a disciplined appraisal of each feature before forming an overall view. NICE NG229 rates four features as white, amber or red, and considers accelerations alongside them:

1. Contractions (frequency per 10 minutes, hypertonus)
2. Baseline Fetal Heart Rate (bpm)
3. Baseline Variability (bpm)
4. Decelerations (timing, duration, recovery, concerning characteristics)
+ Accelerations (presence is reassuring)

1. Uterine Contractions

Assessed with the tocodynamometer and confirmed by abdominal palpation:

• White: fewer than 5 contractions in 10 minutes.
• Amber: 5 or more contractions in 10 minutes with reduced resting time between contractions, or hypertonus (a contraction lasting more than 2 minutes). Excessive contraction frequency reduces intervillous perfusion, so NICE advises a full risk assessment and action to reduce frequency.

2. Baseline Fetal Heart Rate

The mean fetal heart rate when stable, excluding accelerations and decelerations, over a 10-minute period, compared with earlier recordings:

• White: a stable baseline of 110 to 160 bpm.
• Amber: a rise of 20 bpm or more since the start of labour or the last hourly review; a baseline of 100 to 109 bpm (continue usual care if this has been stable, with normal variability and no variable or late decelerations); or an undeterminable baseline.
• Red: a baseline below 100 bpm or above 160 bpm. In the active second stage, a rise of 20 bpm or more is treated as a red feature.
  • Causes of a rising baseline or tachycardia: maternal pyrexia, chorioamnionitis (infection), evolving fetal hypoxia (sympathetic compensation), maternal dehydration, beta-agonists (e.g. terbutaline) and fetal tachyarrhythmia.
  • Causes of bradycardia: acute hypoxia, cord compression or prolapse, maternal hypotension (often after epidural top-up), placental abruption, uterine rupture, rapid descent, and congenital heart block (associated with maternal anti-Ro antibodies). Lower baselines are expected post-term.

3. Baseline Variability

The difference between the highest and lowest heart rate in a 1-minute segment between contractions, excluding accelerations and decelerations:

• White: 5 to 25 bpm, reflecting an intact, oxygenated autonomic nervous system.
• Amber: fewer than 5 bpm for 30 to 50 minutes, or more than 25 bpm for up to 10 minutes.
• Red: fewer than 5 bpm for more than 50 minutes, more than 25 bpm for more than 10 minutes, or a sinusoidal pattern.
• Interpretation: Short periods of reduced variability are normal during fetal sleep. Opioids can reduce variability, but other causes such as sepsis must still be considered. NICE advises urgent review and considering expedited birth if variability is below 5 bpm for more than 30 minutes with risk factors, or below 5 bpm together with other CTG changes such as a rising baseline. Absent variability is very concerning and calls for a low threshold for expedited birth.
• Sinusoidal Pattern: a smooth, regular, wave-like pattern with no normal variability. It signifies severe fetal anaemia (e.g. fetomaternal haemorrhage, red cell alloimmunisation, parvovirus B19) or severe hypoxia.

4. Accelerations

Transient increases in fetal heart rate of 15 bpm or more lasting 15 seconds or more:

• The presence of accelerations, even with reduced variability, is generally a sign that the baby is healthy, and fetal acidosis is unlikely.
• The absence of accelerations on an otherwise normal CTG does not indicate fetal acidosis.
• In the second stage, apparent accelerations are most likely to be the maternal pulse, so maternal and fetal heart rates must be differentiated at least every 5 minutes.

5. Decelerations

Transient falls in fetal heart rate of more than 15 bpm below the baseline lasting 15 seconds or more (decelerations may be shallower when variability is reduced). NICE asks clinicians to describe them as early, variable or late, and not to use the terms "typical" and "atypical":

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Categorisation of CTG Traces (NICE NG229)

Each of the four features is rated white, amber or red:

The overall trace is then classified:

1. Normal: all four features are white. Continue CTG and usual care (or return to intermittent auscultation if CTG was started only because of an IA concern and there are no ongoing risk factors), with a full risk assessment at least hourly.
2. Suspicious: any one feature is amber. Perform a full risk assessment and maternal observations, consider underlying causes and start conservative measures. If there are additional risk factors such as slow progress, sepsis or meconium, obtain an urgent obstetric or senior midwife review and consider fetal scalp stimulation or expediting birth.
3. Pathological: any one feature is red, or two or more are amber. Obtain urgent obstetric and senior midwife review, exclude acute events, start conservative measures, and if the trace remains pathological, consider expediting birth.

Categorisation is a communication tool. NICE stresses that it must be used together with antenatal and intrapartum risk factors and the change in the trace over time, with a fresh "big picture" assessment at least every hour.

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Types of Fetal Hypoxia

Physiology-based teaching describes how fetal hypoxia evolves on the CTG. These patterns help explain why features change, alongside the NICE categorisation:

                      ┌────────────────────────────────────────┐
                      │     PATTERNS OF INTRAPARTUM HYPOXIA    │
                      └───────────────────┬────────────────────┘
                                          │
         ┌────────────────────────────────┼────────────────────────────────┐
         ▼                                ▼                                ▼
┌─────────────────┐              ┌─────────────────┐              ┌─────────────────┐
│  ACUTE HYPOXIA  │              │ SUBACUTE HYPOXIA│              │ EVOLVING HYPOXIA│
│                 │              │                 │              │                 │
│ • Prolonged     │              │ • Decelerations │              │ 1. Loss of      │
│   deceleration  │              │   spend >50% of │              │    accelerations│
│   ≥3 minutes    │              │   time below    │              │ 2. Emergence of │
│ • Catastrophic  │              │   baseline      │              │    decelerations│
│   events:       │              │ • Tachysystole /│              │ 3. Rising       │
│   abruption,    │              │   hypertonus    │              │    baseline     │
│   cord prolapse,│              │ • Acidosis can  │              │ 4. Reduced or   │
│   rupture       │              │   develop fast  │              │    lost         │
│ • Birth if it   │              │                 │              │    variability  │
│   lasts 9 min   │              │                 │              │                 │
└─────────────────┘              └─────────────────┘              └─────────────────┘

1. Acute Hypoxia: a sudden prolonged deceleration or bradycardia lasting 3 minutes or more. It usually results from an acute event such as cord prolapse, placental abruption, uterine rupture, severe maternal hypotension or uterine hyperstimulation. NICE NG229 advises urgent obstetric review, excluding or treating the cause, preparing for urgent birth with neonatal support, and expediting birth if the bradycardia persists for 9 minutes (or sooner if there are significant risk factors). If the heart rate recovers before 9 minutes, the decision is reassessed.
2. Subacute Hypoxia: the fetus spends more than half of the time decelerating, often because contractions are too frequent, leaving too little recovery time between them. Acidosis can develop relatively quickly, so reducing contraction frequency is the priority.
3. Gradually Evolving Hypoxia: the classic sequence over hours as fetal reserves are used up:
   • Step 1: Loss of accelerations.
   • Step 2: Emerging late decelerations or variable decelerations with concerning characteristics.
   • Step 3: Progressive rise in the baseline fetal heart rate (catecholamine surge).
   • Step 4: Sometimes a period of increased variability, reflecting autonomic instability.
   • Step 5: Progressive reduction and loss of variability (decompensation).
   • Step 6: Terminal bradycardia.

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Conservative Intrauterine Resuscitation Measures

When there are concerns about the baby's wellbeing, NICE NG229 advises identifying the most likely underlying cause and starting one or more targeted conservative measures:

1. Maternal Position: Encourage the woman to mobilise or adopt an alternative position, and to avoid lying supine. Changing position can relieve aortocaval compression and cord compression and improve uterine blood flow.
2. Hypotension Only: Do not offer intravenous fluids to treat fetal heart rate abnormalities unless the woman is hypotensive or has signs of sepsis. If hypotension follows an epidural top-up, start IV fluids, move her into the left lateral position and call the anaesthetist, who may give a vasopressor (for example, phenylephrine or ephedrine).
3. Excessive Contraction Frequency: Reduce or stop oxytocin if it is being used. Excessive contractions reduce intervillous blood flow.
4. Tocolysis: Offer a tocolytic drug for excessive contraction frequency; NICE suggests subcutaneous terbutaline 0.25 mg (250 micrograms). Terbutaline is a beta-2 agonist that relaxes the myometrium within minutes and may cause maternal tachycardia.
5. Second Stage: If CTG concerns arise in the active second stage, obtain obstetric review, consider discouraging pushing and stopping any oxytocin to let the baby recover (unless birth is imminent), and agree a plan with time limits for the next review.

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Secondary Diagnostic Tests & Emergency Delivery Thresholds

Digital Fetal Scalp Stimulation (FSS)

If the CTG is suspicious and there are antenatal or intrapartum risk factors, NICE NG229 advises considering digital fetal scalp stimulation during a vaginal examination:

• Reassuring Response: An acceleration in the fetal heart rate with a sustained improvement in the CTG trace. Monitoring of the fetal heart rate and the clinical picture then continues.
• Absence of Response: The absence of an acceleration is a worrying sign that fetal compromise may be present, and expedited birth may be necessary.

Fetal Blood Sampling (FBS)

In its 2022 update, NICE NG229 states that it is unable to make a recommendation about fetal blood sampling because of limited evidence. Some units still use FBS under local protocols, so midwives may meet it in practice:

• Traditional pH Interpretation: 7.25 or above normal; 7.21 to 7.24 borderline; 7.20 or below abnormal.
• Traditional Lactate Interpretation: 4.1 mmol/L or below normal; 4.2 to 4.8 mmol/L borderline; 4.9 mmol/L or above abnormal.
• Usual Contraindications: maternal blood-borne infection (HIV, hepatitis B or C, herpes simplex), fetal bleeding disorders, and prematurity (generally under 34 weeks).

Emergency Delivery Thresholds

If the CTG remains pathological after conservative measures, or an acute bradycardia persists, birth is expedited:

• Acute Bradycardia or Prolonged Deceleration of 3 Minutes or More: urgently seek obstetric review and prepare for urgent birth; expedite the birth if it persists for 9 minutes, or sooner if there are significant risk factors. If there has been an acute event (cord prolapse, suspected abruption or uterine rupture), expedite birth immediately.
• Category 1 Caesarean Section: immediate threat to the life of the woman or baby, with a national decision-to-delivery target of 30 minutes or less.
• Assisted Vaginal Birth: if the cervix is fully dilated and the fetal head is engaged at or below the ischial spines, vacuum or forceps birth may be quickest, in the room or in theatre.
• Documentation: NICE NG229 requires the time that urgent review was sought and the time the decision to expedite birth was made to be recorded.