Obstetrics
Invasive prenatal diagnosis
Fetal genetic abnormalities can be diagnosed prenatally only after specific genetic analysis is performed on a sample of fetal origin in a specialized laboratory.
Obtaining a fetal sample is difficult in prenatal life and it usually requires some kind of an invasive procedure.
Two methods of invasive prenatal procedures are most often used - chorion villous sampling and amniocentesis. They are performed in the first (between 11-13+6 w.g.) and second (16-20 w.g.) trimester, respectively.
Today these procedures still represent the "gold standard" of prenatal diagnosis of the most common fetal chromosomal abnormalities.
Unlike prenatal screening, invasive prenatal tests are diagnostic procedures. They do not assess the risk or probability of a certain condition (for example Down syndrome), rather than they confirm or exclude with certainty the presence or absence of Down syndrome or some other chromosomal abnormality.
The main indications for performing invasive prenatal diagnostics at MC MARKOVS are:
- Advanced maternal age (over 40 years);
- Increased fetal nuchal translucency >3.0 mm between 12-13+6 w.g.;
- Absence/hypoplasia of the fetal nasal bone between 12-13+6 w.g. or in later gestation;
- Presence of other significant ultrasound marker during the first trimester scan or later;
- Increased risk for chromosomal fetal abnormality from the combined biochemical screening between 12-13+6 w.g. or between 16-19 w.g.;
- Positive result from non-invasive prenatal testing (NIPT);
- Presence of some rare inherited genetic syndromes in the family;
- Presence of structural fetal abnormalities established during the first trimester scan or in later gestational age.
All invasive diagnostic procedures are performed under constant ultrasound guidance with the help of a special needle and a syringe through the abdomen of the mother. In this way, a sample is taken from the precursor of the placenta (chorion villous sampling) or from the amniotic fluid (amniocentesis).
The invasive procedure is relatively painless, it takes about one minute to perform and practically is not associated with any risks or hazards for the mother. The sample is sent for laboratory analysis. The results are usually ready in about 1 to 3 weeks. When DNA (PCR) analysis is used the most common chromosomal fetal abnormalities can be diagnosed or ruled out in a shorter period of time – up to 7 working days.
Invasive diagnostic procedures increase the risk of miscarriage by about 0.5-1%. This means that in a group of about 100-200 pregnant ladies who have underwent the procedure, there will be one spontaneous abortion related to the manipulation. For this reason, invasive prenatal diagnosis is performed only after a written informed consent.
The main differences between an amniocentesis and a chorionic villous sampling are as follows:
- Gestational age - chorionic villous sampling is performed in the first trimester while amniocentesis in the second trimester (after 16 w.g.);
- Obtained sample - in chorionic villous sampling, the precursor of the placenta (chorion) is examined, while in amniocentesis - the amniotic fluid.
In about 1% of cases after a chorionic villous sampling, it is impossible to make a definite prenatal diagnosis, due to the presence of the so-called placental mosaicism. This requires performing an amniocentesis in the second trimester (after 16 years).
For more information about the possibility of performing invasive prenatal diagnosis at MC MARKOVS, you can contact your doctor or call at the registration desk.
