Fetomaternal microchimerism occurs during pregnancy and shortly after giving birth for most women. However, not all women who have had children contain fetal cells. Studies suggest that fetomaternal microchimerism could be influenced by killer-cell immunoglobin-like (KIR) ligands. Lymphocytes also influence the development of persisting fetomaternal microchimerism since natural killer cells compose about 70% of lymphocytes in the first trimester of pregnancy. KIR patterns on maternal natural killer cells of the mother and KIR ligands on the fetal cells could have an effect on fetomaternal microchimerism. In one study, mothers with KIR2DS1 exhibited higher levels of fetomaternal microchimerism compared to mothers who were negative for this activating KIR.