Invasive prenatal diagnosis
Examination of fetal tissue during pregnancy (invasive prenatal diagnosis):
The aim of prenatal diagnosis is to determine the health status and level of development of the fetus. According to the method of investigation we distinguish prenatal noninvasive tests (ultrasound, blood tests) and invasive tests, which directly investigate fetal tissue obtained by insertion (invasion) of the sampling instruments to the fetus.
Sampling and testing of the placental tissue (CVS) and amniotic fluid (AMC) are the most common examinations.
- chorionic villus sampling (tissue from which the placenta arises) is performed between 10th and 13th week of pregnancy with needle puncture through the abdominal wall under ultrasound guidance. The sampling needle avoids both the fetus and amniotic fluid. Local skin anesthesia is necessary before the procedure. About 15 - 20 mg of tissue is collected.
- amniocentesis is performed mostly around the 16th week of pregnancy. Amniotic fluid is a clear liquid containing both free biochemical substances and living cells of the fetus. The collection is performed under ultrasound through the abdominal wall with a very thin needle (about 0.5 mm in diameter). The doctor clearly sees the needle tip throughout the collection, fetal injury is therefore highly unlikely. Pregnant women indicate the level of pain as similar to collection of blood or application of conventional injection. About 15-20 ml of amniotic fluid is collected. This small amount spontaneously replenishes over the next few hours after collection.
Risk of complications from sampling of fetal tissue
The most serious complication of pregnancy associated with sampling is spontaneous abortion. Risk reported in the literature is about 0.5% (1/200) above the basic risk of miscarriage for each pregnancy. However, the actual risk of sampling of fetal tissues is difficult to determine. Basic risk of spontaneous abortion between 16th and 20th pregnancy week is about 1-2% by maternal age, regardless of whether the sampling was carried out or not. Also, it is not possible to form a control group of pregnant women with risk for the fetus who were randomly selected for a control group without the possibility of diagnostic tests.
Fetal tissue obtained by invasive procedure is examined primarily for chromosomal defects. When using the method “QF-PCR”, the findings are the most common chromosomal defects (including Down syndrome) and they are known within 24 hours after collection. The result of a complete chromosome examination using microarray, which is much more accurate than the usual “karyotype” is known in about a week.