  Objective: Non-invasive fetal electrocardiography (NI-FECG) shows promise for
capturing novel physiological information that may indicate signs of fetal
distress. However, significant deterioration in NI-FECG signal quality occurs
during the presence of a highly non-conductive layer known as vernix caseosa
which forms on the fetal body surface beginning in approximately the 28th week
of gestation. This work investigates asymmetric modeling of vernix caseosa and
other maternal-fetal tissues in accordance with clinical observations and
assesses their impacts for NI-FECG signal processing. Methods: We develop a
process for simulating dynamic maternal-fetal abdominal ECG mixtures using a
synthetic cardiac source model embedded in a finite element volume conductor.
Using this process, changes in NI-FECG signal morphology are assessed in an
extensive set of finite element models including spatially variable
distributions of vernix caseosa. Results: Our simulations show that volume
conductor asymmetry can result in over 70% error in the observed T/QRS ratio
and significant changes to signal morphology compared to a homogeneous volume
conductor model. Conclusion: Volume conductor effects must be considered when
analyzing T/QRS ratios obtained via NI-FECG and should be considered in future
algorithm benchmarks using simulated data. Significance: This work shows that
without knowledge of the influence of volume conductor effects, clinical
evaluation of the T/QRS ratio derived via NI-FECG should be avoided.
