Dr Wrishi Raphael
Having identified that the pregnancy is high risk for the fetus, tests to determine fetal wellbeing should be started after the 30th week. In the past 10 years, biophysical tests to determine fetal wellbeing have superseded biochemical tests. These tests are:
1] Fetal movement (fetal kick) counts
2] Cardiotocography or CTG
3] Serial ultrasound examinations
4] Doppler flow velocity wave forms.
However none of these has a high positive predictive value. Positive predictive value is the probability that subjects with a positive screening test truly have the disease. But each procedure has a high negative predictive value and the negative predictive value is the probability that subjects with a negative screening test truly don't have the disease.
Cardiotocography (CTG) measures the baby's heart rate. At the same time it also monitors the contractions in the womb (uterus). CTG is used both before birth (antenatally) and during labour, to monitor the baby for any signs of distress.
By looking at various different aspects of the baby's heart rate, doctors and midwives can see how the baby is coping. However, according to WHO, available data suggests that when CTG is used as the “non-stress test” or the “contraction stress test”, it can lead clinicians to use unnecessary or inappropriate interventions as a result of intraobserver and interobserver errors associated with its visual interpretation.
A note of caution should be sounded. The authors of Effective Care in Pregnancy and Childbirth point out that, although the tests may provide 'a minimum level of care and attention in settings where these are adequate', in other settings their use may result in 'a variety of unwarranted interventions', It should be added that unwarranted interventions may lead to an increase in obstetric interventions and possible medicolegal problems. The doctor may be 'damned if he does and damned if he does not'.
It is essential that the doctor explains to the patient what is intended and why, including the limitations of the tests.
How does cardiotocography work?
CTG uses sound waves called ultrasound to detect the baby's heart rate. Ultrasound is a high-frequency sound that you cannot hear, but it can be sent out (emitted) and detected by special machines.
Ultrasound travels freely through fluid and soft tissues. However, ultrasound bounces back as 'echoes' (it is reflected back) when it hits a more solid (dense) surface. For example, the ultrasound will travel freely through blood in a heart chamber. But, when it hits a solid valve, a lot of the ultrasound echoes back. Another example is that when ultrasound travels though bile in a gallbladder it will echo back strongly if it hits a solid gallstone.
So, as ultrasound 'hits' different structures in the body, of different density, it sends back echoes of varying strength.
In CTG monitoring, a special type of ultrasound, called Doppler, is used. This type of ultrasound is used to measure structures that are moving, making it useful for monitoring heart rate.
The other plate on the CTG measures how tense the mother's tummy (abdomen) is. This measurement is used to show when the uterus is contracting.
Interpretation of the CTG:
A suspicious tracing merits closer surveillance and/ or delivery if the fetus is at term, a pathological tracing intensive surveillance and, if persistent, consideration of delivery (frequently by caesarean section) if the fetus has reached viability.
The writer can be reached at: [email protected]