Embryonic and fetal stem cells are not obtainable from your body. It is possible to use these stem cells cultured from laboratories or donors; however, the practice remains highly controversial. Stem cells produced in vitro refer to the lab cultured, artificially produced cells. Donors, on the other hand, contribute stem cells produced in vivo—these cells are derived from natural fertilization. Using allogenic stem cells (stem cells derived from a donor from other than yourself) can trigger an immune response from within the body, as these cells contain different cellular markers from your own body and can be identified as foreign invaders. More so, embryonic stem cells and fetal stem cells prior to five weeks of gestation may uncontrollably proliferate and result in teratomas (germ cell tumors) as a result of unclear differentiation. Adequate differentiation can only be obtained from natural development during pregnancy.