Fetal Heart Rate Monitoring

Fetal heart rate monitoring runs through every labor ward, the hospital setting where Candace Cooley worked alongside office practice while providing gynecologic surgery to inpatients and outpatients. The guidance on how to use it starts with a distinction that surprises many parents. A teaching review of cardiotocography from the World Federation of Societies of Anaesthesiologists summarizes the British national recommendation: for women at low risk of complications, the baby’s heart should be checked by intermittent listening, and continuous electronic tracing is reserved for particular situations.² Those situations include meconium in the waters, a fever in the mother, fresh bleeding in labor, a heart rate outside the normal range or slowing after a contraction when heard by ear, the start of an epidural, and the woman’s own request.²

The public version of that guidance, as given by NHS inform in Scotland, has the midwife listening every 15 minutes in the first stage of labor and after most contractions in the second. It states plainly that continuous monitoring adds little in most uncomplicated labors, that reading the trace is skilled work, and that even very experienced obstetricians can disagree about whether a reading is normal.⁵

Figure

Two ways of checking the baby's heart in labor

	Intermittent listening	Continuous electronic monitoring
Tools	Pinard stethoscope or handheld Doppler	Ultrasound transducer on the abdomen or an electrode on the baby's scalp
How often	Every 15 minutes in the first stage, after most contractions in the second	Without a break
Usual setting	Uncomplicated labor	Labor with complications or known concerns
Movement	Free between checks	Wired, unless a telemetry unit is available

From the ear trumpet to the electronic trace

Listening came first. A history of fetal heart assessment in a nursing journal records that methods of assessing the fetal heart stayed essentially the same for about 150 years, with auscultation standards dating from 1848. Inventors tried to measure labor long before that changed: balloons placed in the uterus in 1872, an external pressure hood in 1896, and in 1906 the first electronic recording of a fetal heartbeat.¹ The first commercial monitor suitable for clinical use was sold in 1968, and adoption was fast. By 1979 at least half of all laboring women were being monitored electronically. The authors argue that expectations for the machines ran ahead of the research on whether they improved outcomes, and that studies have shown poor agreement between clinicians reading the same tracing, and even between one clinician’s readings on different days.¹

Today the tools sit side by side. NHS inform lists the Pinard stethoscope, a small trumpet pressed to the abdomen; the handheld Doppler, which uses ultrasound to pick up the heartbeat; electronic fetal monitoring, with a transducer on the abdomen or a small electrode clipped to the baby’s scalp; and telemetry, which sends the same signal by radio so that a woman can walk around.⁵ The electronic monitor records two lines at once, the fetal heart rate above and the contractions below, so that each change in the heart can be judged against what the uterus is doing.²

The methods differ in accuracy. A study comparing three monitoring methods in 71 laboring women used the scalp electrode as the reference and found that a newer system reading the fetal heart’s electrical signal through electrodes on the mother’s abdomen tracked it more reliably than external ultrasound, which lost the signal more often, especially in women with a higher body mass index.³ The authors note that when external monitoring fails, clinicians may break the waters to place an internal electrode, which carries a small risk of infection.³

Figure

Reference points on a normal tracing

beats per minute, the normal baseline fetal heart rate

110 to 160

beats per minute of normal beat to beat variability

5 to 15

an acceleration rises more than 15 beats for more than 15 seconds

15 and 15

How a tracing is interpreted

Interpretation follows a fixed checklist. The anaesthesia review lists four elements: the baseline rate, the variability around it, the presence of accelerations and the presence of decelerations.² Variability is the small change from one beat to the next; it can fall during normal fetal sleep as well as when oxygen is short. Accelerations, often with fetal movement, are taken as a sign that the baby is well. Decelerations, temporary dips usually tied to contractions, are classified by shape and timing. A tracing with all four features reassuring is called normal; one feature outside the range makes it suspicious, and two or more, or one clearly abnormal feature, make it pathological.²

Figure

Four kinds of deceleration

1. 01
   EarlyMirror each contraction, start and end with it, and come from pressure on the baby's head.
2. 02
   LateBegin after the peak of the contraction and are linked with reduced blood flow through the uterus.
3. 03
   VariableVary in timing and size and come from squeezing of the umbilical cord.
4. 04
   ProlongedA fall of more than 30 beats lasting at least two minutes, reflecting less oxygen reaching the baby.

Why the heart dips at all is still debated. A commentary in The Journal of Physiology explains that normal labor briefly interrupts the exchange of gases through the placenta with each contraction, and that the typical dips most likely reflect a reflex response to these short spells of low oxygen rather than simple pressure on the head. Healthy babies with good placental blood flow adapt to these repeated brief episodes without injury; the clinical task is spotting when that adaptation is failing. The author adds that this physiology explains the limited predictive value of fine detail in the tracing.⁴

When the pattern is unclear, other tests can help. During labor, MedlinePlus describes fetal scalp pH testing, in which a tiny blood sample from the baby’s scalp shows whether it is getting enough oxygen; a value above 7.25 is normal and one below 7.20 abnormal.⁷ Before labor, the same monitor is used in a nonstress test. MedlinePlus explains that a healthy baby’s heart rate rises from time to time, so a reactive result, with those rises present, is reassuring, while a nonreactive one leads to further testing such as an ultrasound based biophysical profile.⁶ If the rate does not rise on its own, rubbing the belly or a gentle sound may be used to wake a sleepy baby.⁶ Monitoring connects to the rest of labor care, from epidural analgesia, which calls for a period of continuous tracing as it is started, to the decision for a cesarean birth.² Which method suits a given labor is a decision for each woman and the midwives and doctors caring for her.

Fetal heart monitoring has come round to where it started. Listening by ear, the standard since the middle of the nineteenth century, survives in the trumpet shaped Pinard stethoscope named in current guidance, and in many hospitals midwives agree that for a normal labor, especially in its first stage, that kind of periodic listening is enough.⁵