Can a fragile x PCR blood test determine the sex of the fetus? My results say:female, 28 and 29 cgg repeats (negative).

No, just mutation. The sex of the fetus is determined by karyotyping/cytogenetic analysis. This test determines the extent of the trinucleotide repeat mutation using southern blot analysis. By determining the number of cgg repeats on the x chromosome, this method allows for more accurate assessment of risk for premutation carriers in terms of their own risk of these syndromes, and of having affected children.