In the fragile X syndrome, the transition from unmethylated moderate expansions of the CGG repeat (premutations) to methylated large expansions (full mutations) occurs only through maternal transmission. The risk of such transition is highly correlated with the size of the maternal premutation (PM), being very low for small PM alleles (approximately 60 repeats), to 100% for alleles above 100 repeats. The timing of this transition was the object of much speculation. A postzygotic transition was proposed as a preferred model, based on the observation that males with full mutation (FM) have PM in sperm. Analysis of tissues from affected fetuses, including additional data reported here, indicate that such a putative postzygotic transition would have to occur very early in embryogenesis and most likely before determination of germ cell lineage. At least 15% of carriers of a FM show a significant proportion of white blood cells carrying a PM (mutation mosaics). We performed a simulation study showing that, if transition to FM is postzygotic, one should observe a much higher proportion of such mosaics in offspring of mothers with small PMs. This was compared with the actual pattern observed in 212 mutated offspring of 112 PM carrier mothers. We found no effect of maternal PM size on incidence of mosaicism in leucocytes. We propose that this is strong, albeit indirect evidence against a postzygotic transition to FM. A transition at an early morula stage (before day 3) cannot, however, be formally excluded.