Invasive prenatal testing tech­niques

Chorionic villus sampling

Chorionic villi are projections of the placenta. They have the same genetic material (chromosomes) as the fetus, so examining the chorionic villi is like examining the fetus itself. After a local anaesthetic, the doctor uses a special needle, either through the vagina or through the wall of the abdomen, to reach the chorionic villi, guided by ultrasound, and takes a small sample from the area.
Complications of this test occur in less than 1% of cases and include bleeding, infection, premature labour and delayed growth of the fetus. Chorionic villus sampling is done in weeks 10–12 of pregnancy. It has an advantage over amniocentesis, described below, because it is done earlier in pregnancy and gives time for decisions to be made. The preliminary results of chorionic villus sampling are ready in less than a week and the final results in about two weeks. Chorionic villus sampling reveals chromosomal abnormalities such as Down syndrome.

Amnio­cen­tesis

In this test, the doctor uses a needle to pierce the abdomen and the uterus and reach the amniotic sac, the sac in which the fetus lies. With the needle, the doctor takes amniotic fluid, which contains cells from the fetus. These cells are examined and show us whether the fetus has any chromosomal abnormality, such as Down syndrome.
Certain substances in the amniotic fluid can also be measured, which help us to tell whether the fetus’s lungs are mature. The test is done in weeks 16–18 of pregnancy. Complications are very rare and include infection and premature labour. The results of this test are ready about two weeks after it is done.

Umbilical vein sampling

The umbilical vein carries blood from the placenta to the fetus. Using a fine needle, the doctor pierces the wall of the abdomen, then the uterus, and reaches the umbilical cord. The umbilical vein lies in the umbilical cord, so the doctor punctures it and draws blood. The blood drawn is the fetus’s own blood and, as you can imagine, many tests can be done on it, for example:

• Checking whether the fetus may have anaemia and, if it is severe, even giving the fetus a blood transfusion through the needle that was inserted to draw the blood.
• Tests for chromosomal abnormalities.
• Tests to investigate a possible infection of the fetus.

The results of most tests done with this method are ready within 2–3 days. This test is done between weeks 18 and 36 of pregnancy.