BirthSmallTalk is two and a half years old, with over 150 posts. The last few months have seen a bunch of new people join me. (Hello! And thank you for dropping by!) I’ve decided it is time to go back to earlier posts to refresh and update them, and have given the entire blog a fresh look. This is week two of a six week plan to revisit posts covering basic concepts and background information to help understand what is going on with fetal heart rate monitoring.
Most of the things I post about relate to CTG monitoring (sometimes referred to as EFM or electronic fetal monitoring). While many of you no doubt know everything I have written below – it is nonetheless a good idea to check in from time to time and make sure we are all talking about the same thing.
The cardiotocograph or CTG is a recording device used to generate a graph showing two sets of information. The first, the cardio- part of the CTG, is the fetal heart rate. The second set of information, the toco- part of the CTG, relates to the activity of the pregnant woman’s uterus. The changes in each of these are plotted over time so relationships between changes in one can be compared with changes in the other.
The process of using a CTG is known as cardiotocography and the output generated is often referred to as a CTG trace or recording. The regular interpretation of a CTG trace by a skilled practitioner is called CTG monitoring. (It is important to be aware that one or more practitioners must actively work to generate an interpretable CTG trace and apply knowledge to interpret the CTG trace. The CTG machine itself does not monitor the fetus, it merely generates a recording. A clinician does the monitoring.)
There are different ways to gather information about the fetal heart rate and the woman’s uterine activity to generate the graph. A distinction is generally made between “external” recording (where the sensors are placed on the outside of the woman’s body) and “internal” recording (where sensors pass through the woman’s vagina to attach to the fetus or sit within the uterine cavity). It is possible to use any method of recording the fetal heart rate combined with any method of recording uterine activity, with the resulting output still considered as a CTG trace.
Ways to record the fetal heart rate
1. Fetal spiral electrode. A fetal spiral (or scalp) electrode (sometimes called a scalp clip) is attached to the part of the fetus that can be felt through the open cervix. It is a thin spiral shaped wire that is screwed through the surface of the skin (no , it doesn’t “clip” on). This electrode records the electrical signal from the fetal heart, known as the fetal electrocardiogram or ECG.
2. Doppler. The most common way to record the heart rate is to use a Doppler sensor, usually a circular disc, placed on the woman’s abdomen over where the chest of the fetus lies. It is usually held in place with an elastic band wrapped around the body. Doppler technology is used to sense the movement of blood through the fetal heart, generating both a sound and a digital signal that can be counted.
3. External fetal ECG recording. New technology has been developed, and continues to be refined, to record the fetal ECG from sensors placed externally on the woman’s abdomen. This is called non-invasive fetal ECG or Ni-fECG. (Use the search function on the Posts page and you’ll find more about these.)
Ways to record uterine activity
1. Intrauterine pressure monitoring. A thin tube called an intrauterine pressure catheter (or IUPC) is passed through the woman’s cervix, past the fetus, into the fluid filled space in the uterus.
2. Tocodynamometry. A circular disc, the same size as the Doppler monitor of the CTG, is worn on the woman’s abdomen, held in place with an elastic strap. It senses changes in pressure in the underlying tissues.
3. Electrohysterography. This is a new approach to recording uterine activity. A sensor worn externally on the woman’s abdomen records electrical activity generated by the uterine muscle as it contracts.
Telemetry – going wireless
Originally the recording device was connected directly to the CTG machine via wires. Telemetry systems now record the output from sensors wirelessly so the woman does not needed to be physically connected to the CTG machine. Many telemetry systems are also waterproof so the sensors can be worn in the shower or bath.
Most but not all maternity services in high-income countries have at least some telemetry capable machines, but they may not have one for every woman using CTG monitoring. The batteries in the sensors need to be charged from time to time. Be aware: if you plan to use telemetry, it might not be possible to use it for your entire labour and it may not be available at all.
Generating a trace – paper or digital
Originally the CTG trace was printed directly onto a moving piece of paper using ink or thermal printing. The paper speed was set to either 1 cm per minute (this has become the standard in most countries where CTG use has become commonplace), or 3 cm per minute. Improvements in computer technology led to digital CTG monitoring becoming possible. Data collected from the fetus and the pregnant woman can be converted to an electronic signal for display on a computer screen, making it possible to save the CTG permanently as a digital record. Digital data can also be moved and displayed somewhere outside the room. Central fetal monitoring systems display the data in a central location in the birth environment. Remote fetal monitoring systems display data in sites well away from the birth environment – such as an obstetrician being able to look at a CTG trace from home on their phone, or a midwife being able to get a second opinion from someone at another hospital.
Other information that might be present on a CTG trace
- Second twin. Most CTG machines can record heart rate data from more than one fetus simultaneously.
- Fetal movement recording. Some tocodynamometer sensors are designed to detect the presence of fetal movements.
- Maternal heart rate. Some sensors detect this automatically, other systems use a pulse oximeter worn on the woman’s finger.
- Maternal blood pressure. Many CTG machines have a plug that allows a blood pressure cuff to be attached. The machine can be set to automatically check the woman’s blood pressure at regular intervals.
- Notes. Written information can be displayed on the CTG. This provides information of use in interpreting the CTG trace and is common with digital CTG systems. Maternity professionals enter data using a computer interface such as a touch screen or keyboard. Maternity professionals might also hand write on the paper print out or apply a sticker if they don’t have access to a digital CTG.
When the CTG might be used
Research refers to the use of the CTG at three different times:
- Antenatal: CTG monitoring during pregnancy and before labour. This typically involves a period of 20 to 30 minutes of monitoring but may require longer periods of time for sufficient information to be present to decide whether it is normal or not. The term “non-stress test” or NST means the same thing. This is to distinguish it from a “stress test” (rarely used any more) where uterine contractions are stimulated to “stress” the fetus.
- Admission: refers to the generation of a CTG trace at the time a woman presents to a maternity service in labour. Like antenatal CTG monitoring, the trace typically runs for 20 to 30 minutes.
- Intrapartum: refers to the generation of a CTG trace during a woman’s labour. This may be continuous (running until the birth of the baby), or intermittent (for example 20 minutes of tracing performed every second hour).
There are of course many other times when CTG monitoring is used, but these have not been researched. This includes before and after an intervention (like an external cephalic version), or prior to discharge from an episode of professional care.
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Additional reading resources
Argyridis, S., & Arulkumaran, S. (2016). Intrapartum fetal monitoring. In Best Practice in Labour and Delivery (pp. 40-59). https://doi.org/10.1017/9781316144961.006
Arnold, J. J., & Gawrys, B. L. (202). Intrapartum fetal monitoring. American Family Physician, 102(3), 158-167. https://www.ncbi.nlm.nih.gov/pubmed/32735438
Ayres-de-Campos, D. (2018). Electronic fetal monitoring or cardiotocography, 50 years later: what’s in a name? American Journal of Obstetrics and Gynecology, 218(6), 545-546. https://doi.org/10.1016/j.ajog.2018.03.011
Blix, E., Maude, R., Hals, E., Kisa, S., Karlsen, E., Nohr, E. A., de Jonge, A., Lindgren, H., Downe, S., Reinar, L. M., Foureur, M. J., Pay, A. S. D., & Kaasen, A. (2019). Intermittent auscultation fetal monitoring during labour: A systematic scoping review to identify methods, effects, and accuracy. PLoS ONE, 14(7), e0219573. https://doi.org/10.1371/journal.pone.0219573
Hirsch, E. (2019). Electronic fetal monitoring to prevent fetal brain injury: A ubiquitous yet flawed tool. JAMA, 322(7), 611-612. https://doi.org/10.1001/jama.2019.8918
Knupp, R. J., Andrews, W. W., & Tita, A. T. N. (2020). The future of electronic fetal monitoring. Best Practice & Research in Clinical Obstetrics & Gynaecology, 67, 44-52. https://doi.org/10.1016/j.bpobgyn.2020.02.004
- Listening to the fetus: the history of fetal heart rate monitoring
- What is intermittent auscultation?
Tags: Doppler, Electrohysterography, fetal ECG, fetal scalp electrode, Fetal spiral electrode, FSE, Intrauterine pressure catheter, Noninvasive fetal ECG, telemetry
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