From the Guidelines
The answer is b. Affinity of hemoglobin for oxygen. Fetal hemoglobin (HbF) has a lower affinity for oxygen than adult hemoglobin (HbA), which is the opposite of what many people expect. This decreased affinity is due to the presence of fetal hemoglobin (HbF), which contains two gamma chains instead of the beta chains found in adult hemoglobin. This structural difference allows fetal hemoglobin to more readily release oxygen to fetal tissues, facilitating oxygen transfer from maternal to fetal circulation across the placenta.
The other options are incorrect because:
- Fetal hemoglobin concentration is actually higher than maternal levels to compensate for the lower oxygen environment, as seen in the relationship between fetal hemoglobin and gestational age 1.
- Fetal erythrocytes bind more 2,3-BPG than maternal cells, which helps to shift the dissociation curve to the right and increase oxygen delivery to the tissues 1.
- Fetal cardiac output per kg body weight is higher to meet metabolic demands, with a 30–50% increase in cardiac output occurring in normal pregnancy 1.
- Fetal cardiac glycogen content is elevated to provide energy reserves for the heart during birth when oxygen may be temporarily limited.
Overall, the unique properties of fetal hemoglobin allow for efficient oxygen transfer from the mother to the fetus, which is crucial for fetal development and survival.
From the Research
Fetal and Maternal Blood Comparison
The following values can be compared between the fetus and the mother:
- Hemoglobin concentration:
- No direct comparison is provided in the studies, however, it is known that fetal hemoglobin has a higher oxygen affinity than maternal hemoglobin 2.
- Affinity of hemoglobin for oxygen:
- Fetal blood normally has a higher oxygen affinity than maternal blood because of the predominance of hemoglobin (Hb) F in the former and of Hb A in the latter 2.
- Erythrocyte binding of 2,3-BPG:
- Cardiac output/kg body weight:
- No direct comparison is provided in the studies.
- Cardiac glycogen content:
- No direct comparison is provided in the studies.
Values Normally Less in the Fetus
Based on the provided studies, the following value would normally be less in the fetus than in the mother:
- Affinity of hemoglobin for oxygen is higher in the fetus, so the value that is less is not the affinity itself, but rather the effect of 2,3-BPG on hemoglobin, which is less in the fetus due to its lower response to 2,3-BPG 3.
- Erythrocyte binding of 2,3-BPG is less in the fetus due to the lower response of fetal hemoglobin to 2,3-BPG 3.