Antenatal Fetal Monitoring

How does the mother assess fetal wellbeing?

Maternal assessment of fetal wellbeing

Based on premise that fetal activity decreases with increasing intrauterine hypoxia and may cease completely for a variable period before fetal death

In 3rd trimester, the fetus spends 10% of its time making gross fetal body movements

Mother can appreciate 70-80% of these movements

Factors affecting maternal perception of fetal movement

Fetal movements are poorly perceived when women are involved in normal daily activities and are better detected when resting and focused on fetal activity.

The presence of an anterior placenta is also known to reduce maternal perception of movements, but this ceases to be relevant in the third trimester.

Obesity, smoking, use of sedative drugs, and primiparity are associated with reduced perception

Pros

• Free of charge
• Non-invasive
• Easy to implement

Cons

• Subjective
• May cause increased anxiety
• Need for additional visits and tests that may be required when reduced movements are reported.

Fetal Kick chart

It is a method of maternal assessment of fetal wellbeing

Fetal movement decreases with hypoxia

E.g. Cardiff count to ten

• Less than 12 counts in 24 hrs is associated with significant increase in perinatal mortality and is an indication for further testing

Counting of four movements in 1 hour when the woman is resting and focused on detection of movements.

May be affected by placenta location, amniotic fluid, obesity, parity, medications, anxiety

Disadvantage— maternal anxiety

Triggers for further evaluation

• No movement in 12 hours
• < 3 movements in 1 hour for 2 consecutive days
• < 10 movements in 12 hours…Cardiff count to 10

Insufficient evidence to recommend routine fetal movement counting to prevent fetal death (Cochrane review)

Uses external methods of monitoring the fetal heart rate

Interval between successive beats is measured thereby giving a tracing of the fetal heart rate

Also measures frequency of uterine activity

Gives a tracing of the fetal heart rate and that of uterine activity

Antenatal CTG

Usually performed for a period of 20-30 minutes.

Utilizes the CTG machine which has the following

• 2 transducers recording fetal heart rate and uterine activity
• An event marker which the mother is asked to press anytime she feels a fetal movement.

These movements also appear on the tracing together with the fetal heart rate and uterine activity.

Interpreted based on the premise that in a normal (non-hypoxic fetus), there is acceleration of the fetal heart rate in response to fetal movement. This implies that the autonomic nervous system of the fetus is functioning properly and is capable of reacting appropriately to stressors.

A normal NST is said to be reactive after the examination of the tracing for 4 parameters viz

• Baseline heart rate
• Variability
• Accelerations
• Decelerations

Baseline heart rate

Mean level of the most horizontal and less oscillatory FHR segments

It is estimated in 10-minute periods and expressed in beats per minute (bpm).

A fetus has a normal baseline if its value lies between 110 and 160 bpm.

Preterm fetuses tend to have values towards the upper end of this range and post-term fetuses towards the lower end.

Tachycardia: baseline value > 160 bpm lasting more than 10 minutes

• Causes include maternal pyrexia (commonest cause),
• In the initial stages of a non-acute fetal hypoxemia, catecholamine secretion may also result in tachycardia.
• Drugs e.g. beta-agonists like terbutaline, albuterol
• Fetal arrhythmias

Bradycardia: baseline value < 110 bpm lasting more than 10 minutes. Values between 100 and 110 bpm may occur in normal fetuses, especially in post-term pregnancies.

Other causes of bradycardia include

• Maternal hypothermia
• Drugs e.g. beta-blockers like arenolol, propanolol
• Fetal arrhythmias

Baseline variability

Variation of the FHR from one beat to the next.

Occurs as a result of the interaction between the nervous system, chemoreceptors, baroreceptors and cardiac responsiveness

It is a good indicator of how healthy the fetus is at the moment the test is being performed.

A healthy fetus will constantly adapt its heart rate to respond to changes in its environment.

Normal variability is between 10-25 bpm.

Variability is determined by looking at the difference between the peaks and the troughs of the baseline FHR of the CTG tracing.

Causes of reduced variability include fetal sleep, hypoxia, drugs, prematurity, congenital heart disease etc.

Accelerations

Sustained increase in the FHR above the baseline heart rate of >15 bpm for >15 seconds.

The presence of accelerations is reassuring.

Antenatally, there should be at least 2 accelerations every 15 minutes.

A healthy fetus has accelerations in the FHR in response to contractions and fetal movement.

Note: If an acceleration lasts 10 minutes or more, it is regarded as a change in baseline heart rate.

Decelerations

A sustained decrease in FHR below the baseline heart rate of >15 bpm for >15 seconds.

• Early decelerations: caused by fetal head compression and do not indicate fetal hypoxia; have normal variability.
• Variable decelerations (V-shaped) exhibit a rapid drop (onset to nadir in less than 30 seconds), good variability within the deceleration, rapid recovery to the baseline, varying in size, shape and relationship to uterine contractions caused by umbilical cord compression, and on their own are seldom associated with an important degree of fetal hypoxia.
• Late decelerations (U-shaped or with reduced variability) have a gradual onset, gradual return to the baseline (>30 seconds), or reduced variability within the deceleration.

Criteria for a Normal CTG

• Baseline heart rate of 110-150 bpm
• Variability of 10-25 bpm
• 2 accelerations in 20 mins
• No decelerations

It is reassuring to the physician as chance of intrauterine fetal death within 1 week of a normal CTG is 1 per 1000

Suspicious CTG

• Absence of accelerations
• Abnormal baseline rate
• Reduced baseline variability
• Variable decelerations

Abnormal CTG

No accelerations and two or more of the following

• Abnormal baseline rate
• Abnormal variability
• Repetitive late decelerations
• Variable decelerations
• Others
• Prolonged bradycardia

When hypoxia develops gradually, the first features to be noted are

• The absence of accelerations
• Increase in baseline rate
• Reduction in baseline variability

The worsening situation may be reflected by the deceleration getting deeper and wider

Limitations of NST/ antenatal CTG

• Relies solely on fetal heart rate to determine fetal wellbeing
• Reactive NST implies that the fetus is not asphyxiated at the time of testing
• Associated with high false positive rates (50- 75%)
• High proportion of fetuses will be judged to have a non-reassuring status and may be subjected to unnecessary intervention

A.k.a. Oxytocin challenge test

Used if there is a suboptimal NST

Based on premise that uterine contractions result in reduced blood flow in intervillous spaces and a fetus with inadequate reserve will demonstrate recurrent late decelerations in response to hypoxia

Requires uterine contractions of moderate intensity; 3 in 10 lasting 40-60 seconds

Oxytocin infusion is administered to induce contractions

Cardiotocograph is used

Baseline uterine activity should be determined

Moderate uterine contraction lasting 40-60 sec is generated using oxytocin infusion

Contraindicated in—

• Patient at risk for preterm labor e.g.
• Patient with PROM
• Twin gestation
• Incompetent cervix
• Previous classical CS
• Placenta previa

Results of CST

Negative: no late or significant variable deceleration

• Consistently associated with good fetal outcome
• Perinatal death within 1 week is < 1 in 1000

Positive: late decelerations with at least 50% of contractions

• Associated with increased risk of fetal death

Suspicious: intermittent late or variable decelerations

Hyper-stimulation: decelerations with contractions >90 seconds duration or at minute frequency

Unsatisfactory: <3 contractions in 10 minutes or unsatisfactory tracing

Limitation: high false positive (abnormal test result in a normal fetus), unnecessary premature interventions

Helps to perform an in-utero physical examination of the fetus and evaluate dynamic functions reflecting the integrity of the CNS

The more complete the examination of the fetus, its activities and its environment, the more accurate may be the determination of the health of the fetus

Helps to overcome the high false positive rate by of the NST by assessing other parameters of fetal wellbeing

5 parameters are used to determine BPP

1. NST/fetal reactivity
2. Fetal movements or gross body movement
3. Fetal breathing movements
4. Fetal tone
5. Amniotic fluid volume

Involves use of fetal heart rate reactivity and amniotic fluid index

FHR indicates present fetal condition; Amniotic fluid index (AFI) indicates long-term fetal status

Requires about 10 minutes to perform

Has revolutionized the practice of obstetrics

Identifies congenital anomalies e.g. anencephaly, hydrocephaly

Used to determine GA, fetal weight, malposition, abnormal presentation, placental abnormalities, abruptio, placenta previa, IUGR

Doppler, provides continuous tracings of fetal heart activity;

Can identify placenta and fetal blood vessels; assess placental function

Used to assess fetal hemodynamic status by detecting movement of blood in fetal, maternal and placental circulations

Useful in IUGR

In the presence of placental dysfunction and growth restriction, the earliest changes are detected in uterine arteries followed by the umbilical artery and then the middle cerebral artery and lastly the ductus venosus, usually indicates imminent fetal demise.

Noninvasive assessment of fetal, maternal and placental circulation

Umbilical artery velocimetry; determines the degree of blood flow in the umbilical arteries; these depend on uteroplacental circulation

Blood flow slows down in diastole and gets faster in systole

Degree of slowing down is dependent on the resistance in the artery

Reversed end-diastolic flow is associated with increased perinatal mortality

Relies on Doppler Effect which observes changes in the frequency of transmitted waves when relative motion exists between the source and the target

These sound waves are utilised to assess fetal haemodynamic status by detecting movement of blood in the fetal, maternal or placental circulations.

Measurements of blood flow in these vessels, helps to determine the health or otherwise of the fetus, and the timing of delivery.

Vessels that have been studied extensively in relation to this are the uterine artery, umbilical vessels, ductus venosus and middle cerebral artery.

Normally, diastolic flow in umbilical arteries increases (i.e. resistance falls) throughout gestation resulting in improved blood supply to the fetus

Increased umbilical artery resistance implies faulty placental perfusion— fetal hypoxia, IUGR, IUFD