Objective Cell trafficking during pregnancy results in persistence of small populations of fetal cells in the mother, known as fetal microchimerism (FMc). Changes in cell-free fetal DNA during gestation have been well-described, however, less is known about dynamic changes in fetal immune cells in maternal blood. We investigated FMc in maternal peripheral blood mononuclear cells (PBMC) longitudinally across gestation. Study Design Thirty-five women with normal pregnancies were studied. FMc was identified in PBMC, CD4+ and CD8+ subsets employing quantitative PCR assays targeting fetal-specific genetic polymorphisms. FMc quantities were reported as fetal genome equivalents (gEq) per 1,000,000 gEq mother’s cells. Poisson regression modeled the rate of FMc detection. Main Outcome Measure FMc in PBMC Results The probability of detecting one fetal cell equivalent increased 6.2-fold each trimester [Incidence Rate Ratio (IRR) 95% CI: 1.73, 21.91; p=0.005]. Although FMC in PBMC was not detected for the majority of time points, 7 of 35 women had detectable FMc during pregnancy at one or more time points, with the majority of positive samples being from the third trimester. There was a suggestion of greater HLA-sharing in families where women had FMc in PBMC. FMc was detected in 9% of CD4+ (2/23) and 18% of CD8+ (3/25) subsets. Conclusions FMc in PBMC increased as gestation progressed and was found within CD4+ and CD8+ subsets in some women in the latter half of gestation. A number of factors could influence cellular FMc levels including subclinical fetal-maternal interface changes and events related to parturition. Whether FMc during pregnancy predicts persistent FMc and/or correlates with fetal-maternal HLA-relationships also merits further study.