B7. Prenatal diagnostics I. (invasive methods)

From greek.doctor

Prenatal diagnostics are techniques which can be used to detect genetic disorders or birth defects in the foetus. There are three purposes of these:

  • To prepare necessary medical or surgical treatments soon after birth, or even intrapartum
  • To give the parents the chance to abort a foetus with the diagnosed condition
  • To give the parents the chance to prepare for a baby with a certain condition

We distinguish invasive and non-invasive methods. The invasive methods are diagnostic tests and include chorionic villus sampling and amniocentesis. The non-invasive methods are screening tests. Both chorionic villus sampling and amniocentesis yield the same genetic information.

Invasive methods are indicated in the following cases:

  • As confirmation after non-invasive methods suggest a genetic disorder or defect
  • High risk for a certain genetic disorder (family history)

Chorion villus sampling

Chorionic villus sampling (CVS) involves ultrasound-guided transcervical or transabdominal biopsy of a chorionic villus. It is performed in weeks 10 – 13.

Because it’s invasive, it carries a risk of complications, including alloimmunisation, pregnancy loss, and foetal defects.

Amniocentesis

Amniocentesis (AC) involves ultrasound-guided transabdominal acquisition of amniotic fluid. Ultrasound is performed to locate the placenta and a pocket of amniotic fluid away from the foetus. A needle is then inserted through the abdominal wall and uterine wall. AC is performed after week 15.

It may also be used for amnioinfusion, the infusion of lactated Ringer or normal saline into the amniotic cavity, in case of oligohydramnios. In case of polyhydramnios, it may be used to drain excess fluid.

The amniotic fluid can be tested for multiple different compounds:

  • Phosphatidylglycerol
  • L/S ratio (test for foetal lung maturation)
    • Mature lungs give a ratio of 2:1
  • Triple test – AFP, hCG, UE3
  • Foetal maturity index

AC carries a risk of infection and pregnancy loss.

Cordocentesis

Cordocentesis involves ultrasound-guided transabdominal acquisition of foetal blood from the umbilical cord. This is difficult and only performed if CVS or AC is insufficient. It also carries a risk of infection and pregnancy loss.