page_title: Percutaneous umbilical cord blood sampling
section_title: Procedure
hierarchical_section_title: Percutaneous umbilical cord blood sampling / Procedure
context_page_description: Percutaneous umbilical cord blood sampling, also called cordocentesis, fetal blood sampling, or umbilical vein sampling is a diagnostic genetic test that examines blood from the fetal umbilical cord to detect fetal abnormalities. Fetal and maternal blood supply are typically connected in utero with one vein and two arteries to the fetus. The umbilical vein is responsible for delivering oxygen rich blood to the fetus from the mother; the umbilical arteries are responsible for removing oxygen poor blood from the fetus. This allows for the fetus’ tissues to properly perfuse. PUBS provides a means of rapid chromosome analysis and is useful when information cannot be obtained through amniocentesis, chorionic villus sampling, or ultrasound; this test carries a significant risk of complication and is typically reserved for pregnancies determined to be at high risk for genetic defect. It has been used with mothers with immune thrombocytopenic purpura.
context_section_description: If the fetus is viable, the procedure is performed close to an operating room in case an emergency cesarean section is necessary due to complications caused by the procedure.  Currently, there is no definite age of viability because this depends on the fetus’ ability to survive outside the womb, which in cases of premature births, can depend on access to medical care and technology needed to keep the fetus alive through the neonatal stage.  Fetal viability typically occurs at about 24 to 25 weeks of gestation.  When the fetus is in between the ages of 24–34 weeks, a glucocorticoid is given to the patient about 24 hours before the procedure to stimulate lung maturity.  An ultrasound is performed before the procedure to view the position of the fetus and may be used during the procedure to help guide the needle.  The mother’s blood is drawn for comparison against fetal blood, and intravenous access is established in the mother in order to supply medications as needed.  To reduce the risk of intraamniotic infection, antibiotics are supplied through the intravenous access about 30–60 minutes before the procedure.  If movement of the fetus is a risk to the success of the procedure, the fetus may be paralyzed using a fetal paralytic drug.
  A 20 or 22 gauge spinal needle is typically used in PUBS and may be prepared with an anticoagulant, which helps to reduce the risk of clot formation. During the procedure, the first step is to locate a relatively stable segment of the umbilical cord.  A typical sampling site would be where the segment of the umbilical cord is closest to the placenta.  However, there is a risk of maternal blood contamination at this site.  Blood sampling may be achieved with more ease if the placenta is in the anterior position.  However, if the placenta is in the posterior position, the fetus might block direct access to the umbilical cord.  Once the umbilical cord is reached and the correct position of the needle is confirmed, the fetal blood is drawn.  The needle is removed after all necessary samples are taken.  The site of puncture is monitored after the procedure for bleeding.  Also, if the fetus is viable, fetal heart rate is monitored post-procedure for one to two hours.
  After the blood samples are obtained, they are placed into tubes containing anticoagulants in order to stop the blood from clotting.  If the blood sample was obtained at the site close to the placenta, a fetal blood confirmation test should be done to ensure no mixing of fetal and maternal blood occurred before the diagnostic tests are done on the blood.  Fetal red blood cells (RBC) are usually bigger than maternal RBCs, and the average volume of RBCs, the mean corpuscular volume (MCV), is one of the methods used to determine whether or not the fetal blood has been contaminated.  Another method, human chorionic gonadotropin (hCG) determination, can detect maternal blood because maternal blood has high levels of hCG.  The hemoglobin alkaline denaturation test (Apt test) can detect the presence of maternal blood, which is indicated by a color change from red to brown when the sample is added to alkali reagent.  Blood typing would also detect maternal blood, as the I antigen only occurs in adults.  The Kleihauer–Betke test can detect very small amounts of maternal blood before the third trimester of pregnancy by monitoring hemoglobin elution in acid because adult and fetal hemoglobin elute differently in acid.  Finally, a white blood cell count can detect maternal blood in the sample, as fetal white blood cells are primarily leukocytes, while maternal white blood cells are mostly neutrophils.  If amniotic fluid infiltrated the sample, then there would be a reduction in the volume of RBCs, white blood cells, and platelets in the sample.   Also, patterns consistent with amniotic fluid would be visible in the sample.
caption: This image shows anterior blood sampling from the umbilical cord.