What parameter is normally lower in the fetus than in the mother during the second trimester: hemoglobin concentration, affinity of hemoglobin for oxygen, hemoglobin binding of 2,3-bisphosphoglycerate (2,3-BPG), cardiac output per kilogram body weight, or cardiac glycogen content?

From the Guidelines

The answer is b. Affinity of hemoglobin for oxygen. Fetal hemoglobin (HbF) has a lower affinity for 2,3-BPG compared to adult hemoglobin (HbA), which results in fetal hemoglobin having a higher affinity for oxygen than maternal hemoglobin 1. This physiological adaptation facilitates oxygen transfer from maternal to fetal circulation across the placenta. Fetal hemoglobin's increased oxygen affinity creates a favorable gradient for oxygen movement from mother to fetus. The other options are incorrect because:

• Fetal hemoglobin concentration is actually higher than maternal levels (not lower) 1
• Fetal hemoglobin binds less 2,3-BPG (not more) 1
• Fetal cardiac output per kg body weight is higher than in adults to meet increased metabolic demands 1
• Cardiac glycogen content is greater in the fetus as a reserve for the stress of birth This difference in hemoglobin oxygen affinity is a crucial adaptation that ensures adequate oxygenation of fetal tissues despite the relatively hypoxic intrauterine environment. Key points to consider:
• Fetal development and maternal maintenance of pregnancy require multiorgan physiological adaptations 1
• The oxyhemoglobin dissociation curve is shifted to the right in the mother during pregnancy, and to the left in the fetus, conferring relative resilience to hypoxic conditions 1
• Fetal hemoglobin concentration and binding properties are critical for ensuring adequate oxygenation of fetal tissues 1