Percutaneous Umbilical Blood Sampling (PUBS)
This, quite simply (but technically tricky), involves the placement of a needle into the umbilical vein to remove fetal blood for a variety of testing purposes such as: chromosomal analysis; genetic studies; hemoglobin analysis (hemoglobinopathies); fetal anemia; platelet count; assessment of fetal acid-base status; fetal infection; coagulation system abnormalities; immune deficiencies. It is commonly used when rapid (48-72hr) and precise fetal chromosomal studies are indicated; in the definitive assessment of anemia under conditions of isoimmunization or parvovirus infection (and as a route of intrauterine transfusion if anemia is confirmed); in the evaluation of thrombocytopenia (= low platelets) resulting from either autoimmune or alloimmune (analogous to Rh-disease) conditions; in the evaluation of fetal hydrops; and, in the confirmation of fetal infections.
PUBS is performed under aseptic conditions and direct ultrasound guidance, much like amniocentesis. Without going into great detail, a slightly larger bore needle is often used than that used with amniocentesis, especially if the procedure is being done for suspected fetal anemia and a transfusion via intravenous access is warranted. When technically feasible, we prefer to access the umbilical vein within 1-2 cm of its placental insertion site. Unlike with amniocentesis, many of us actually prefer to go through the placenta to get to this site. This provides a more stable site for insertion than free cord floating around in the amniotic fluid, improves the prospects for success and probably reduces the risks of the procedure. (Fetal blood sampling can at times also be done directly from the baby, the most common sampling sites being the intrahepatic vein and the fetal heart, but special preparations and precautions must be taken under these circumstances and do not warrant discussion herein).
Although at times the fetal condition might justify earlier evaluation, most fetal blood sampling procedures are done in babies that are 23 weeks or beyond. Since we now consider almost all babies at this gestational age potentially viable, it is recommended that the procedure be performed at an institution that can handle extremely premature babies, and/or babies that may be compromised by the medical condition that led to the PUBS, and at a location in which an emergency cesarean section can readily be performed if a complication arises during or in the immediate post-procedure period.
The most common complications include fetal bradycardia (slow heart rate) during the procedure, hemorrhage or obstructing blood clot at the needle insertion site, and intrauterine infection. Fetal bradycardia is often transient and its mechanism is unclear, but it may be related to spasm in the muscles of the uterine arteries if these were inadvertently punctured during the procedure. Prolonged bradycardia is more common, and more ominous, if it occurs when a baby is severely anemic, hypoxic, acidotic, hydropic (in heart failure), or is hemorrhaging uncontrollably from either the umbilical vein or an umbilical artery. If this cannot be corrected by our typical intrauterine resuscitative measures, immediate delivery may be indicated if the baby is at a gestational age where viability is possible.
Significant fetal hemorrhage from the needle insertion site is a relatively uncommon event; however, it is more likely to occur when the baby has low platelets as may be found in autoimmune, alloimmune, or parvovirus-induced thrombocytopenia. Of these, the greatest risk is in alloimmune thromobocytopenia because the platelet counts are often dangerously low and the platelets that are present may not function normally in clot formation. Babies affected by alloimmune thrombocytopenia are also at great risk for intracranial hemorrhage remote from the labor and delivery process.
In my experience, introduction of infection at the time of the procedure, either from maternal skin bacteria or blood borne organisms such as HIV, CMV, and hepatitis viruses, is a very uncommon event. Rupturing fetal membranes also occurs infrequently, but slightly more often than with amniocentesis alone. Risks for any of these complications rises with the length and complexity of the procedure, the larger the bore of needle used, and maternal obesity.
There are theoretical risks of PUBS to the mother such as causing sensitization to fetal blood cells or platelets, causing damage to internal organs, introducing infection, or causing bleeding, but these are also very uncommon. Probably the greatest risk is that of an emergency cesarean section if this is required to manage a fetal complication.
Procedure-related fetal loss rate is difficult to ascertain but is probably in the range of 1-2% and again is related to the complexity of the procedure and the fetal problem that led to the procedure being recommended in the first place. Babies with an indication for PUBS may be critically ill and, for example, those with hydrops, especially nonimmune hydrops, may not be salvageable regardless of any interventions attempted. It is difficult to consider a loss of one of these babies to be a ‘procedure-related event’ when their morbidity is so high at the outset.
PUBS is the riskiest and most challenging of the more common invasive diagnostic procedures we perform, but it is usually reserved for the most severe fetal compromise as well. In recent years, one of the more common indications for PUBS, screening for fetal anemia, has been replaced by the noninvasive Doppler flow assessment of peak systolic velocity in the fetal middle cerebral artery. PUBS is now only done under these circumstances when a significant fetal anemia is suggested by an abnormal Doppler flow result. Also, with the recent advances in genetic and molecular technologies, many of the studies that required PUBS in the past can be performed simply on amniotic fluid and/or the cells contained within it. And, quite likely, many of these studies will be able to be performed on the small amounts of fetal cells and DNA contained in maternal blood specimens as these technologies continue to advance, further reducing the need for these invasive studies.