A 28-year-old G2P1 woman in her 10th week of gestation presents to an outpatient psychiatric clinic with her partner due to increasingly erratic behavior over the past five weeks. Her partner reports she has been experiencing profound insomnia, sleeping approximately 1 to 2 hours nightly, and often appears to be conversing with unseen individuals. She has accumulated significant credit card debt from impulsive online purchases, primarily for expensive baby gear. Prior trials of various first and second-generation antipsychotics, initiated by her previous physician, have failed to mitigate her symptoms. Her vital signs today are: temperature 37.10C, blood pressure 130/85 mmHg, pulse 92/min, respirations 16/min, and oxygen saturation 99% on room air. During the examination, she exhibits pressured speech, psychomotor agitation, and an elevated, euphoric mood, stating, "I feel fantastic, like I'm finally reaching my full potential, and this baby is going to be incredibly special." She denies any history of substance abuse or suicidal ideation. Given her refractory presentation, the consulting psychiatrist considers initiating a mood stabilizer known for its efficacy in severe, treatment-resistant cases. Considering this patient's presentation and the potential medication prescribed, what is the most probable maternal psychiatric diagnosis, and what is the primary fetal cardiac complication requiring careful monitoring?