A prenatal diagnostic test for genetic abnormalities and infection. Percutaneous umbilical cord blood sampling (PUBS) is one of the newest methods of sampling fetal blood for testing. The procedure is used only after the 17th week of pregnancy, and only when other diagnostic procedures have not yielded a definitive result. PUBS is exclusively performed by specially trained doctors because it is a complicated and difficult procedure. Guided by ultrasound, the doctor uses a sterile needle to take blood from the umbilical cord. PUBS is associated with more complications, such as injury to the umbilical cord, than either amniocentesis or chorionic villus sampling. PUBS is performed late in a pregnancy to diagnose conditions such as genetic abnormalities and to determine if a harmful infection has been passed from the mother to the fetus. It can also detect life-threatening anemia in the fetus that is caused by Rh incompatibility, and the same procedure can then be used to give the fetus a blood transfusion directly into the umbilical cord.