11 Monitoring FHR and FMP Using Ultrasound
The maternal heart rate may simulate a normal fetal heart rate pattern (i.e., it may mask a
FHR deceleration or fetal demise).
Especially during pushing with contractions in the second stage of labor, the maternal heart rate
may increase to the point where it may equal or exceed the fetal rate. Here the maternal trace may
mimic a normal fetal trace while the fetus may be having decelerations or fetal demise has
occurred. This change from fetal to maternal heart rate pattern may not be at all obvious unless
CCV is used and represents the most dangerous pitfall of all the artifacts because fetal distress may
The maternal heart rate may simulate a FHR acceleration, which is an increase of the fetal
During expulsive efforts, the maternal heart rate normally accelerates and may be at or above the
normal FHR range.
The FHR may display gradual appearing decelerations. Generally, the "false decelerations"
described above are abrupt. Rarely, combinations of "noisy/erratic signal" associated with changes
in maternal and/or fetal rate or movement will produce more gradual appearing "false
decelerations" but these are usually short-lived with an abrupt return to an obviously stable FHR
"Noisy/Erratic" signals: With mixed or weak signals the tracing may reveal very brief episodes of
erratic recorded traces. These represent the auto correlation algorithm finding brief sequences of
apparent and persistent heartbeats amidst a mixed or weak signal. These erratic recorded traces are
commonplace, especially in association with fetal or maternal movement. During prolonged periods
of such noisy/erratic signals, the fetus is not being adequately monitored.
Drop out: With mixed or weak signals there may be no heart rate tracing at all. These episodes reflect
that if the algorithm does not find an apparent and persistent heartbeat amidst a mixed or weak signal,
it will not print a heart rate on the tracing. Brief episodes of drop out are commonplace, especially in
association with fetal or maternal movement. During prolonged periods of drop out, the fetus is
not being adequately monitored.
With multiple fetuses, the potential to experience these artifacts is increased. Positioning of the
transducer is even more critical. Ultrasound scanning should be used to help with positioning of
individual transducers. See also "Monitoring Twin FHRs" on page 159 and "Monitoring Triple FHRs"
on page 167.
Obtaining a Good Heart Signal
To successfully position the ultrasound transducer, first determine the fetal position using palpation.
Position the transducer over the strongest audible fetal heart sound from the monitor's speaker and
wait at least six seconds after each transducer adjustment to verify a good signal quality displayed
on the Signal Quality Indicator and a consistent FHR numeric display. Having determined the position
that provides a strong fetal signal, fix the transducer on the abdomen with the belt.
If the quality of the signal or the appearance of the heart rate trace from the ultrasound transducer is
questionable, the transducer should be repositioned as described above. Alternatively, the use of an
ultrasound scanner will greatly facilitate the determination of the optimal site for the ultrasound heart
rate transducer. Factors during the second stage of labor that may influence the quality of the FHR
tracing obtained with ultrasound include:
Changing contour of the maternal abdomen