The fetus, like an adult, has a circulatory system that passes blood cells throughout the blood vessels in the body. Unlike adults, fetal circulation has not fully matured and shows several circulatory changes that will be explained in the following article.
While in adults gas exchange occurs in the lungs (carbon dioxide is replaced by oxygen), fetal circulation occurs through the placenta – an organ that provides nutrients from the mother.
In fetoplacental circulation, blood flows from the placenta to the fetus through the umbilical cord. From there, the enriched blood flows to the heart. Most of the blood bypasses the lungs (that are not in use while the fetus is developing in the uterus) and is directed to the rest of the organ systems. It is then sent back through the umbilical cord for waste to be eliminated in the mother’s circulation. The fetus’ blood composition is also different prenatally, hemoglobin is adjusted to the fetus’ physiological needs during pregnancy.
What is percutaneous umbilical cord blood sampling?
Percutaneous umbilical blood sampling (also known as PUBS or cordocentesis) is an invasive procedure that samples blood from the fetus. Unlike regular blood tests, the fetal circulatory system is not directly accessible for testing, and this complex procedure is required to sample fetal blood. In this test, a needle is inserted into the umbilical cord (or another blood vessel) where a sample of fetal blood is drawn.
When is the test needed?
Fetal blood sampling can provide genetic information concerning the fetus, such as fetal blood count. This information is usually derived from tests with a lower risk, such as maternal blood testing, amniocentesis, or chorionic villus sampling (CVS).
When these tests do not provide adequate information, or when they cannot be performed for various reasons, cordocentesis may be used. This invasive procedure is associated with several risks, for this reason it is used only on rare occasions, when no alternative is available.
PUBS results can provide information on metabolic diseases, diseases such as hydrops fetalis, infections, and certain structural anomalies.
How is the test performed?
Cordocentesis usually obtains blood from the umbilical cord. Blood vessels in the umbilical cord are considered large and accessible and can easily be discerned in a Doppler ultrasound scan.
The procedure is usually performed in proximity to an operating room, to enable an emergency cesarean section if necessary.
Before the procedure, steroid injections are usually given to women in weeks 24-34 into pregnancy. This is done to help develop the fetus’ lungs in case an emergency c-section is required. The test is not usually performed before week 17 of pregnancy, due to the vulnerability of the umbilical vein before this stage.
Like many other prenatal tests, this procedure is performed under ultrasound direction. The gynecologist performing the procedure identifies the fetal veins from which the sample will be taken, after cleansing the abdomen.
In most cases, local anesthetic for the mother is sufficient. Occasionally, fetal anesthesia is also required to prevent unnecessary fetal movements during the procedure. The needle is inserted guided by an ultrasound scan. The test does not take very long and under skilled hands is usually completed within an hour.
After the sample is drawn, certain laboratory techniques are used to ensure the sample indeed contains fetal blood (and not maternal blood). These techniques identify fetal components in the blood and include testing the hormone hCG, an Apt test, and testing for I antigens that are only found on fetal blood cells.
In addition, a Kleihauer–Betke test is used to identify fetal blood and test for the presence of maternal blood in the sample. Various indices of fetal white and red blood cells may also assist in correctly identifying the fetal blood in the sample.
The main complication of cordocentesis is pregnancy loss, or miscarriage. The risk of miscarriage is around 1.5%-3%, depending on the stage the procedure is performed (the risk is higher in more advanced stages of pregnancy – after week 28). More complications may include fetal-maternal bleeding, changes in fetal heart rate, infections, and preterm birth.