Fetal progenitor cells can persist in the blood streams of mothers up to twenty-seven years after birth.

This article explains that women who give birth can retain fetal stem cells in their blood stream for as long as 27 years after birth. This was proved by testing both pregnant and non-pregnant women who had given birth to males previously, and male DNA was detected in many of the women. The conclusion is that pregnancy may establish a long-term, low-grade chimeric state in the human female.