Fetal Blood

Fetal blood comes from the embryo.  At about 27 days the baby starts to create blood from their Hematopoetic blood cells. The blood type in the fetus is determined by the childs' genes. For example, a parent with Heterozygous type A blood mixed with another Heterozygous type B blood can have any kind of blood type. That combination can mean that there is a 25% chance of AB blood, 25% chance of A blood, 25% chance of B blood, and 25% chance of O blood. Whether or not the blood type is positive or negative is also genetic. In this case positive blood is the dominant type of blood and negative is recessive. A problem may arise with an Rh-negative mom having a Rh-positive baby. If a small amount of the baby’s blood mixes with mom's blood, their body may respond as if it were allergic to the baby and make antibodies that can cross the placenta. The mom's antibodies will  attack the baby’s blood. They break down the fetus’s red blood cells. This condition can become severe enough to cause serious illness, brain damage, or even death in the fetus or newborn. Normally, the mother's blood doesn't mix with the babies because of the placenta. However, It is common for the blood to mix in delivery though. This has little to no affect on that particular child but the antibodies will have been created by the mom. Rh antibodies are harmless until the mother's second or later pregnancies. If she is ever carrying another Rh-positive child, her Rh antibodies will recognize the Rh proteins on the surface of the baby's blood cells as foreign, and pass into the baby's bloodstream and attack those cells.

http://www.ncbi.nlm.nih.gov/pubmed/15906238
http://www.quia.com/jg/2502993list.html
http://americanpregnancy.org/pregnancycomplications/rhfactor-2.html
http://umm.edu/health/medical/pregnancy/specialcare-pregnancies/blood-group-rh-incompatibility