Background During pregnancy, mother-child interactions trigger a variety of subtle changes in the maternal body, which may be reflected in the status of peripheral blood mononuclear cells (PBMCs). Although these cells are easy to access and monitor, a PBMC atlas for pregnant women has not yet been constructed. Methods We applied single-cell RNA sequencing (scRNA-seq) to profile 198,356 PBMCs derived from 136 pregnant women (gestation weeks 6 to 40) and a control cohort. We also used scRNA-seq data to establish a transcriptomic clock and thereby predicted the gestational age of normal pregnancy. Results We identified reconfiguration of the peripheral immune cell phenotype during pregnancy, including interferon-stimulated gene upregulation, activation of RNA splicing-related pathways and immune activity of cell subpopulations. We also developed a cell-type-specific model to predict gestational age of normal pregnancy. Conclusions We constructed a single-cell atlas of PBMCs in pregnant women spanning the entire gestation period, which should help improve our understanding of PBMC composition turnover in pregnant women.