Memory B cells (MBCs) epitomize the adaptation of the immune system to the environment. We identify two MBC subsets in peripheral blood, CD27 and CD27 MBCs, whose frequency changes with age. Heavy chain variable region (VH) usage, somatic mutation frequency replacement-to-silent ratio, and CDR3 property changes, reflecting consecutive selection of highly antigen-specific, low cross-reactive antibody variants, all demonstrate that CD27 and CD27 MBCs represent sequential MBC developmental stages, and stringent antigen-driven pressure selects CD27 into the CD27 MBC pool. Dynamics of human MBCs are exploited in pregnancy, when 50% of maternal MBCs are lost and CD27 MBCs transit to the more differentiated CD27 stage. In the postpartum period, the maternal MBC pool is replenished by the expansion of persistent CD27 clones. Thus, the stability and flexibility of human B cell memory is ensured by CD27 MBCs that expand and differentiate in response to change.