Fetal macrocytosis in association with chromosomal abnormalities.

Mean red cell volume (MCV) was determined in 264 fetuses between 15-41 weeks. After exclusion of anemic, hypoxic, and chromosomally abnormal fetuses, the MCV in 208 umbilical venous samples was shown to decrease with gestation (r = 0.64; P less than .001), and a normal range was constructed by linear regression analysis. An elevated MCV was found in both fetuses with triploidy, in four of five with monosomy X, and in four of ten with trisomies 18 or 21. The MCV was similarly raised in four of five fetuses with gross anomalies in whom cytogenetic cultures had failed. Significant correlations were found in chromosomally abnormal fetuses between the elevation in MCV and both the nucleated red cell (r = 0.69; P less than .01) and reticulocyte counts (r = 0.57; P less than .05). There was a similar correlation with nucleated red cells in 16 severely anemic fetuses with Rh disease, 12 of whom had a raised MCV. Elevation in MCV was unrelated to hypoxia. Macrocytosis had a sensitivity of 71% and a specificity of 95% in the second trimester for predicting an abnormal karyotype in nonanemic fetuses (kappa index 0.60). Fetal MCV may provide clinically useful information while one awaits culture results. We suggest that karyotyping be considered in fetuses undergoing blood sampling for other indications in whom the MCV is raised.