The CDR3 of the Ig H chain (CDR3H) is significantly different in fetal and adult repertoires. To understand the mechanisms involved in the developmental changes in the CDR3H of Ig H chains, sets of nonproductive VHDJH rearrangements obtained from fetal, full-term neonates and adult single B cells were analyzed and compared with the corresponding productive repertoires. Analysis of the nonproductive repertoires was particularly informative in assessing developmental changes in the molecular mechanisms of VHDJH recombination because these rearrangements did not encode a protein and therefore their distribution was not affected by selection. Although a number of differences were noted, the major reasons that fetal B cells expressed Ig H chains with shorter CDR3H were both diminished TdT activity in the DJH junction and the preferential use of the short JH proximal D segment D7-27. The enhanced usage of D7-27 by fetal B cells appeared to relate to its position in the locus rather than its short length. The CDR3H progressively acquired a more adult phenotype during ontogeny. In fetal B cells, there was decreased recurrent DJH rearrangements before VH-DJ H rearrangement and increased usage of junctional microhomologies both of which also converted to the adult pattern during ontogeny. Overall, these results indicate that the decreased length and complexity of the CDR3 H of fetal B cells primarily reflect limited enzymatic modifications of the joins as well as a tendency to use proximal D and JH segments during DJH rearrangements.