There are two routes for retrieving fetal blood: Placing the needle through the placenta or through the amniotic sac. The placenta's position in the uterus and the spot where it connects to the umbilical cord determine which method your doctor uses.
If the placenta is attached toward the front of the uterus (placenta anterior), he inserts the needle directly into the umbilical cord without passing through the amniotic sac. The amniotic sac, or "bag of waters," is the fluid-filled structure that cushions and protects the developing fetus.
If the placenta is attached toward the back of the uterus (placenta posterior), the needle must pass through the amniotic sac to reach the umbilical cord. This may cause some temporary bleeding and cramping.
You should receive Rh immune globulin (RHIG) at the time of PUBS if you are an Rh-negative unsensitized patient.