Chorionic Villus Sampling
Commonly known as CVS, the procedure involves procuring a tiny bit of the developing placental tissue. This is done by inserting a needle through the mother’s tummy, uterus and into the developing placenta – all the time under the guidance of ultrasound. A numbing injection is first given to the mother’s tummy.
The procedure is generally safe: in healthy pregnancies, the procedure related pregnancy loss is less than 0.5% in experienced hands. The tissue obtained is then processed for various genetic tests according to the requirement for that particular case.
The most common reason for performing CVS is to rule out Down syndrome: this may be because there was a previous child with Down Syndrome, the mother’s age is more than 35 years, or the Down syndrome screening test showed a high chance for Down Syndrome. In other instances, CVS is done to know the chromosomes of the fetus that has physical defects or to know if the fetus has the same gene level defect that is present in another family member (e.g thalassemia).
The test is usually performed until the 14th week after which the ‘yield’ is low. No special preparation is required from the patient’s part: take a good bath, have a simple breakfast, and come in a comfortable loose dress. After the paperwork, the actual procedure would take about 20 minutes following which bed rest is advised for another 20 – 30 mins. You may return home after a few hours. Following the procedure, we advise to avoid heavy work for about a week. About 2% of women may experience slight spotting in the couple of days following the procedure and this should not cause major concern. If there are persistent bleeding, leaking, fever, cramps then you should consult your obstetrician without delay.
The reasons for performing an amniocentesis test are essentially the same as that for doing a chorionic villus sampling. The amnio test is done after 15 weeks. The safety and complications of amnio test are also similar to CVS in experienced hands.
Also referred to as ‘amnio’, this is a simple procedure to obtain a small amount of amniotic fluid (about 20mL) by inserting a special needle through the mother’s tummy and into the uterus. The needle will always be under ultrasound guidance so that it does not harm the fetus.
Choosing between the two tests is a matter of preference by the physician: CVS has the advantage of early testing while amnio has the advantage of being widely available due to its technical simplicity.
Fetal Blood Sampling
In certain situations, testing the fetal blood directly is required. Commonly, when there is a fetal condition called hydrops that results in accumulation of fluid inside the fetus, fetal blood sampling is required to find out the cause. In other situations, the reasons for testing is essentially the same as that of chorionic villus sampling or amniocentesis. Generally, when the testing has to be done beyond 20 weeks, FBS is preferred over amniocentesis.