There are a variety of non-invasive and invasive techniques available for prenatal diagnosis. Each of them can be applied only during specific time periods during the pregnancy for the greatest utility.

The three methods discussed here include, ultrasonography, amniocentesis and chorionic villus sampling.

Ultrasonography

This is a non-invasive procedure that is harmless to both the fetus and the mother.

1. High frequency sound waves are utilized to produce visible images from the pattern of the echos made by different tissues and organs, including the baby in the amniotic cavity.

2. The developing embryo can first be visualized at about 6 weeks gestation.

3. Recognition of the major internal organs and extremities to determine if any are abnormal can best be accomplished between 16 to 20 weeks of gestation.

Although an ultrasound examination can be quite useful to determine the size and position of the fetus, the size and position of the placenta, the amount of amniotic fluid, and the appearance of fetal anatomy, there are limitations to this procedure.

Limitations of Ultrasonography

Subtle abnormalities may not be detected until later in pregnancy, or may not be detected at all. A good example of this is Down syndrome (trisomy 21) where the morphologic abnormalities are often not marked, but only subtle.

Amniocentesis

This is an invasive procedure in which a needle is passed through the mother's lower abdomen into the amniotic cavity inside the uterus.

This can only be done between 14-20 weeks of gestation as the right amount of amniotic fluid is required for this procedure to be done.

For prenatal diagnosis, an ultrasound examination always proceeds amniocentesis in order to determine gestational age, the position of the fetus and placenta, and determine if enough amniotic fluid is present.

Why is amniotic fluid necessary?

1. Within the amniotic fluid are fetal cells (mostly derived from fetal skin) which can be grown in culture for chromosome analysis, biochemical analysis, and molecular biologic analysis.

2. In the third trimester of pregnancy, the amniotic fluid can be analyzed for the determination of fetal lung maturity.

3. This is important when the fetus is below 35 to 36 weeks gestation, because the lungs may not be mature enough to sustain life following birth in some cases.

4. The amniotic fluid can be analyzed by looking for an appropriate number of lamellar bodies. Other tests for fetal lung maturity include fluorescence polarization (fpol), lecithin: sphingomyelin (LS) ratio, and phosphatidyl glycerol (PG).

Risks and Limitations of amniocentesis

These tests have poor positive predictive value for respiratory distress, so the decision to do amniocentesis can be made by consideration of issues around gestational age and urgency of delivery.

Risks with amniocentesis are uncommon but include fetal loss and maternal Rh sensitization.

The increased risk for fetal mortality following amniocentesis is about 0.5% above what would normally be expected.

Chorionic Villus Sampling (CVS)

In this procedure, a catheter is passed via the vagina through the cervix and into the uterus to the developing placenta under ultrasound guidance.

An alternative approach is transabdominal. The introduction of the catheter allows sampling of cells from the placental chorionic villi.

hese cells can then be analyzed by a variety of techniques. The most common test employed on cells obtained by CVS is chromosome analysis to determine the karyotype of the fetus.

The cells can also be grown in culture for biochemical or molecular biologic analysis.

CVS can be safely performed between 9.5 and 12.5 weeks of gestation.

Risks of CVS

CVS has the disadvantage of being an invasive procedure, and it has a small but significant rate of morbidity for the fetus; this loss rate is about 0.5 to 1% higher than for women undergoing amniocentesis.

There is also the possibility that maternal blood cells in the developing placenta will be sampled instead of fetal cells and may modify the results of chromosome analysis.