THEamniocentesis is the most common test used for prenatal diagnosis. It aims to take a little amniotic liquid in which bathes the fetus. This liquid contains cell and other substances capable of giving key information about the health of the fetus. 

Amniocentesis is usually done between the 14th and 20th week of pregnancy to detect chromosomal abnormality (mainly Down Syndrome or Trisomy 21) as well as certain congenital malformations. It can be practiced:

• when the mother’s age is advanced. From the age of 35, the risks of birth defects are higher.
• when blood tests and first trimester ultrasound indicate a risk of chromosomal abnormality
• if a chromosomal abnormality exists in the parents
• when the baby has abnormalities on the ultrasound of the 2^st quarter

When performed, amniocentesis also allows the sex of the fetus to be determined.

The examination can also take place later, during the third trimester of pregnancy:

• to see if the fetus has developed lungs
• or to detect an infection of the amniotic fluid (in the event of growth retardation, for example).

The examination is performed by an obstetrician in the hospital. He first checks the position of the fetus and the placenta by performing an ultrasound. This will serve as a guide during the operation.

The exam involves inserting a needle through the belly and uterus. Once in the amniotic sac, the doctor withdraws about 30 ml of fluid and then withdraws the needle. The puncture site is covered with a bandage.

The entire test takes about 15 minutes, and the needle only stays in the uterus for a minute or two.

Throughout the examination, the doctor checks the heart rate of the fetus, as well as the mother’s blood pressure and breathing.

Note that the mother’s bladder must be empty before performing the examination.

The amniotic fluid is then analyzed to:

• to establish the karyotype for chromosome analysis
• to measure certain substances present in the liquid, such as alpha-fetoprotein (to detect the possible existence of a malformation of the nervous system or the abdominal wall of the fetus)

Amniocentesis is an invasive examination that can present two main risks:

• miscarriage, in around one in 200 to 300 (depending on the center)
• infection of the uterus (rare)

A 24 hour rest is advised after the test. It is possible to feel abdominal cramps.

Analysis times vary depending on the laboratory. Most often, it takes 3-4 weeks to get the fetal karyotype, but it can be faster.

If the number of cells obtained and analyzed is sufficient, the conclusions of chromosomal studies are almost absolutely reliable.

In the event of an abnormality detected, the couple will have the choice of continuing the pregnancy or asking to terminate it. It is a difficult decision that is up to them exclusively.