Total and differential leucocyte counts were measured in cord blood samples obtained by cordocentesis (n=316) or at elective caesarean section (n= l1) from normal fetuses of between 18 and 40 weeks' gestation. The total fetal leucocyte count increased exponentially from 2*8X109/l at 18 weeks to 11*8X109/l at term. The lymphocyte and monocyte counts increased linearly and the number of neutrophils increased exponentially from a mean value of 0.2 x 109/l at 18 weeks to 0-8 x 109/l at 31 weeks and then 8-5x109/l at term. Early myeloid cells, eosinophils, and basophils were observed in 24%, 55%, and 15% of the blood films respectively; they contributed less than 2% to the total leucocyte count and there were no significant changes with gestation. The physiological leucopenia observed in fetuses early in the third trimester may partly explain the predisposition of premature neonates to infection.

The fragile X mutation is diagnosed from the structure of the FMR1 gene in blood cell DNA. An estimated 12 to 41% of affected males are mosaics who carry both a “full mutation” allele from which there is no gene expression and a “premutation” allele which has normal gene expression. We compared the DNA in blood cells and skin fibroblasts from four mosaic fragile X males to see if there was a difference in the relative amounts of premutation and full mutation alleles within the tissues of these individuals. Two of these males showed striking differences in the ratio of premutation to full mutation in different tissues while the other two showed only slight differences. These observations conform with the widely accepted hypothesis that the fragile X CGG repeat is unstable in somatic tissue during early embryogenesis. Accordingly, the mosaicism in brain and skin, which are both ectodermal in origin, may be similar to each other but different from blood which is not ectodermal in origin. Thus, the ratio of full mutation to premutation allele in skin fibroblasts might be a better indicator of psychological impairment than the ratio in blood cells. © 1996 Wiley‐Liss, Inc.

Two brothers with fra(X) positive X-linked mental retardation (XLMR) were treated with folic acid. Initially a double blind cross-over design was employed followed by a long-term high dose trial. A decrease in the frequency of fra(X) positive cells was observed when low folic acid culture medium was used but not when an FUdR induction system was employed. Selected behavioral characteristics improved in both while receiving folic acid. Decreased hyperactivity, greater attention span, increased motor coordination, increased quantity and quality of speech were noted. Improvement in Leiter mental age and regression after cessation of treatment was seen in one subject but not in the other. Further controlled trials with larger numbers of subjects using high doses of folic acid over longer periods of time are needed to assess the possible benefits of this experimental form of treatment.

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