This fetus was the product of a 43-year-old mother with Rh-immunization. She received a single intrauterine transfusion of 20 mL of red blood cells at gestational week 31.1. The weight of the fetus was approximately 1,300 g during the transfusion. Fetal hemoglobin represented 90% of hemoglobin before the transfusion. Hemoglobin concentration and hematocrit values before and after transfusion were 12.6 and 14.0 g/dL, and 38% and 42%, respectively. Consequently, the calculated red cell iron increased by 4.9 mg (1,300 g× 4% × 8.5% × 1.1). Therefore, of the 15.4 mg (20 mL × 70% × 1.1) of iron given to the fetus, about 10.5 mg (15.4 - 4.9 mg) was incorporated into tissue stores rather than into hemoglobin. The serum transferrin receptor concentration, which is a measure that reflects both the rate of erythropoiesis and iron status, rose from 4.0 to 4.6 mg/L. The serum ferritin rose from 44 to 48 μg/L. The transferrin receptor/ferritin ratio varied between 90.0 and 95.8 (μg/μg).

Very little is known about transferrin receptor levels during fetal life. In this fetus with hemolytic disease, and in six others studied, the transferrin receptor values are close to postnatal values. One would have expected to find highly elevated values due to the very active erythropoiesis. Our calculations indicate that a major portion of given red blood cells does not benefit oxygen transport unless the blood volume is much larger than anticipated.