Cordocentesis – also called fetal blood sampling, Percutaneous Umbilical Cord Blood Sampling (PUBS), and umbilical vein sampling – is a highly specialized diagnostic test performed in the second trimester to check for genetic defects in the developing baby.
During a cordocentesis, a doctor will remove a blood sample from the baby’s umbilical cord. (This is why this diagnostic procedure is often referred to as fetal blood testing, or fetal blood sampling).
A cordocentesis can only be performed after 18 weeks of pregnancy. It is too risky to perform this fetal blood test before this week in gestation, because the umbilical cord has not adequately developed yet.
Why is a Cordocentesis Test Performed?
Because of the slightly high risk of miscarriage (compared to other diagnostic tests), doctors only perform cordocentesis when chorionic villus sampling (CVS), amniocentesis, and ultrasounds have failed to provide the information they need. Sometimes, a CVS or amniocentesis gave inconclusive results. So, fetal blood sampling will be the only other option to learn if the baby has a genetic defect.
A cordocentesis test isn’t performed very often, since CVS and amniocentesis are safer diagnostic tests.
When fetal blood sampling is done, it is used to identify:
- Chromosomal Abnormalities – If there is a chromosomal defect in the baby, like Down syndrome, fetal blood testing can diagnose this genetic defect.
- Infections in the Baby – When the mother has a serious infection, like rubella (German measles) or toxoplasmosis, fetal blood sampling can be used to determine whether or not the infection has passed to the baby. Rubella can cause major birth defects in the developing baby (a defect called congenital rubella syndrome). When passed to an unborn baby, toxoplasmosis can cause stillbirths and long-term physical and neurological damage.
- Intrauterine Growth Restriction – Sometimes, a baby does not grow properly in the womb. A cordocentesis test can be used to determine what is causing your baby to experienced restricted growth in utero.
- Rh Factor – A majority of pregnant women have Rh-positive blood. However, if you are Rh-negative and you need to know your baby’s Rh factor, a cordocentesis can provide the answers. Sometimes when a pregnant woman has Rh-negative blood, and her baby is Rh positive, her body can produce antibodies that attack the developing baby’s red blood cells. This can lead to a serious pregnancy complication.
- Blood Disorders – Cordocentesis can also determine if the baby has a blood disorder, like thalassemia, sickle cell disease, anemia, or fetal hemolytic disease.
Cordocentesis has a high degree of accuracy. It does have drawbacks, however. For example, it cannot tell you the severity of any genetic defects or other abnormalities. It also cannot identify neural tube defects.
What Happens During Cordocentesis (Fetal Blood Sampling)?
The cordocentesis procedure is very similar to an amniocentesis. However, instead of the doctor retrieving amniotic fluid from your womb, he or she is withdrawing fetal blood from the umbilical cord.
The entire procedure takes about an hour, and you can get results within 72 hours. Cordocentesis is usually performed at your doctor’s office, but it can also be done at a hospital.
You may be given antibiotics shortly before the procedure begins, which helps lower your risk of getting a uterine infection.
- During the fetal blood sampling procedure, you will lie on your back.
- An ultrasound will be used to monitor your baby’s exact location in the womb.
- Your belly will be cleaned with an antiseptic.
- The doctor will insert a thin, hollow needle through the wall of your abdomen and into your uterus.
- Umbilical cord blood will be collected into a syringe.
- The needle will be removed, and the cordocentesis is complete.
Some pregnant women will experience cramping when the needle first enters their uterus. It’s also common to experience some light vaginal bleeding after the umbilical blood sample is taken.
In very rare cases, the healthcare provider will not be able to get a blood sample from your baby’s umbilical cord. When this happens, the doctor will try to take a blood sample from a vein in the unborn baby’s liver.
Risks and Complications after a Cordocentesis
Because fetal blood sampling is considered an invasive procedure, it does come with a variety of risks. Miscarriage is one of the risks.
A woman’s risk of a miscarriage is about 2 in 100 after a cordocentesis procedure. This is slightly higher than the miscarriage risk associated with some of other prenatal diagnostic tests available (like amniocentesis and chorionic villus sampling).
Other risks include:
- Bleeding from the location where the needle entered your abdomen.
- Uterine infection
- The temporarily drop in your baby’s heart rate
- Premature rupture of the membranes
Since cordocentesis does come with these risks, you may want to weigh the risks versus the benefits of the prenatal diagnostic test.
After fetal cord blood sampling (cordocentesis), if you notice any chills, fever, or leakage of amniotic fluid, contact your doctor immediately.