Small amounts of genetically foreign cells (microchimerism, Mc) traffic between a mother and fetus during pregnancy. Commonly, these grafts durably persist. For women, multiple naturally acquired Mc grafts can accrue, as they harbor Mc from their own mothers (maternal Mc, MMc) and subsequently acquire fetal Mc (FMc) through pregnancy. The nature of interactions between these naturally acquired grafts may inform, and be informed by, observations in transplantation, including the effect of noninherited maternal HLA antigens (NIMA) and double-unit cord blood transplantation (CBT). We asked whether FMc and MMc are impacted by the addition of new grafts as evaluated by increasing parity. Mc was identified by quantitative PCR for a nonshared polymorphism unique to the Mc source. Despite increasing sources of Mc, FMc did not increase with increasing parity. MMc concentration was significantly lower with increasing parity. The odds ratio for detection of MMc for 2 or more births compared with 1 birth was .11 (95% CI 0.03-0.42, P = .001). These observations suggest that interactions occur among naturally acquired grafts and are of interest in light of recent observations of graft-graft interaction resulting in predominance of 1 unit in double-unit CBT and the correlation of MMc with the NIMA effect.