Hypoplastic left heart syndrome (HLHS) is a congenital heart disease characterized by the underdevelopment of left heart structures. Previous studies using lumped-parameter models focused on extreme cases where the left ventricle is absent. We propose a model that examines HLHS severity through the gradual degradation of left heart functions and morphology. This includes simulating birth and oxygen transport, resulting in pressure-volume loops for healthy and HLHS cases in fetal and neonatal stages, as well as the transients of the blood flow rate, pressure, and oxygen saturation in the system. Notably, oxygen saturation levels post-birth show a non-linear response to the degree of hypoplasia right after birth, making the prediction of neonatal oxygen levels from fetal conditions a potential biomarker for patient classification in uncertain neonatal cases.