Amniocentesis is a pregnancy test used to detect certain conditions in the baby, such as Down syndrome. It’s not exactly a routine test because it carries some risks. Due to its purpose, it is categorized as a diagnostic test, which is used to confirm suspicions. It is also called “chorionic villus biopsy.” The exam involves taking a sample of the amniotic fluid from the uterus and analyzing it. It is only recommended when there are abnormalities in routine tests, such as the morphological ultrasound done in the first trimester, or the non-invasive prenatal diagnosis.
What is amniotic fluid?
Amniotic fluid is found inside the well-known water sac that breaks at the baby’s birth. This sac can be described as a membrane filled with fluid, shaped like a pouch (the amnion), which “holds” the baby during pregnancy1. The amount of fluid in the sac increases as the baby develops. In general, by the 28th week of pregnancy, the fluid usually reaches its maximum, with a volume of about one liter.
The importance of amniotic fluid is that it cushions blows or other traumas that a woman might suffer to the belly, while also preventing strong temperature variations. This protects the fetus as it develops and keeps it warm—two things fundamental for normal development. Later, the amniotic fluid allows the baby more mobility inside the uterus, which strengthens their developing muscles and bones.
The fluid contains various substances, fetal cells with the same DNA as the baby, and some microorganisms. Therefore, this material can clarify many questions about the baby’s health.
What conditions does amniocentesis detect?
Generally, testing the amniotic fluid serves to diagnose genetic diseases or neurological malformations2. The conditions that the test is able to identify are:
- Chromosomal abnormalities, such as Down syndrome, Edward syndrome, Patau syndrome, Turner syndrome, or Klinefelter syndrome;
- Congenital neural tube diseases, such as spina bifida and anencephaly;
- Hereditary blood diseases, such as sickle cell anemia, thalassemia, and hemophilia;
- Cystic fibrosis;
- Muscular dystrophy.
Other information that amniocentesis can reveal, but that can also be diagnosed in other less invasive ways, includes the baby’s sex, whether there is Rh factor incompatibility between mother and child (when the baby’s blood is Rh positive and the mother’s is Rh negative), or if there is an infection. It is also possible to determine the child’s paternity before birth through a DNA test on the amniotic fluid. However, the test is not usually used for this purpose.
There are other suspected developmental issues in the baby that cannot be detected by amniocentesis but by more detailed ultrasounds, such as cases of cleft lip.
When should amniocentesis be done?
Amniocentesis is recommended by the doctor when there are changes in early pregnancy tests, combined with risk factors from family history—that is, when there are cases of congenital or genetic abnormalities in the mother’s or father’s family.
Amniocentesis should be performed starting from the 15th week of pregnancy. This is because, as this week begins the second trimester, the amniotic fluid already has enough volume in the uterus for doctors to collect a suitable sample with less risk of miscarriage or complications in the baby’s development.
There are some signs and risk factors that may lead the doctor to suspect formation problems in the baby and know when to perform amniocentesis. The most common include:
- Pregnancy after age 35;
- Abnormalities in routine tests in the first trimester, such as blood tests or ultrasounds;
- Family history of genetic diseases;
- Inadequate conditions for fetal intrauterine growth;
In cases of malformations, amniocentesis is, in some countries, a way to diagnose early pregnancy problems and allow for abortion in these cases. In Brazil, however, the law currently is not permissive with malformation or genetic conditions, except in cases of anencephalic babies or when the woman’s life is at risk. In this context, the importance of the exam is more restricted to giving the family early knowledge of the child’s health issue.
In less severe cases, prior knowledge of the baby’s clinical conditions facilitates preparation for childbirth; the family may hire a neonatal ICU team, for example, if financially able. It is also important for the psychological preparation of the family that will welcome a child with special needs.
How is the procedure done?
Amniocentesis is a simple and quick test, lasting about 20 minutes. It is usually done in the obstetrician’s office. It works as follows: the amniotic fluid test requires the insertion of a long, very fine needle into the abdomen, to reach inside the uterine cavity.
The expectant mother lies down while the doctor uses ultrasound images to guide them, check the baby’s position, and locate the amniotic fluid sac. After inserting the needle into the sac, the specialist extracts a small amount of fluid: about two tablespoons.
There is a possibility of using local anesthesia. However, many specialists believe this is unnecessary. Either way, you can talk to your obstetrician about your concerns before the exam and whether you need anesthesia. The pain from the needle prick or the discomfort of the exam is usually mild and depends greatly on the woman’s sensitivity.
One tip to ease nervousness is to focus on the ultrasound images of your baby, not on the procedure itself. Here is the step-by-step procedure:
- With the woman lying on her back and with gel applied, the doctor uses an ultrasound device to locate the exact position of the baby in the uterus.
- The abdominal skin is sterilized with an antiseptic.
- The doctor inserts the amniocentesis needle through the skin and abdominal wall until it reaches the uterine cavity and the surface of the amniotic fluid sac—only on its surface, not touching the baby!
- A small amount of amniotic fluid is aspirated with a syringe, and the needle is finally removed from the uterus.
IMPORTANT: It is best to remain still while the needle is being inserted and the amniotic fluid is being extracted. Always follow the doctor’s instructions.
What takes the longest is the ultrasound procedure. The fluid aspiration itself only takes five minutes. The doctor must observe the baby’s heart and watch for uterine contractions during the procedure to ensure no stress from this invasive method.
Patients report a burning sensation when the needle penetrates the skin and cramps or spasms when it reaches the uterus. At the end of the procedure, women may also feel mild cramping. If there is uncomfortable pain, an analgesic may be prescribed. Specialists recommend resting for at least 24 hours after the test, and sexual activity should be avoided for seven days.
Test cost
The cost of amniocentesis in private clinics, including lab analysis and the medical procedure, varies from one thousand to over two thousand reais, and can be more expensive if done in the 3rd trimester of pregnancy. Therefore, it is usually cheaper to perform it in the 2nd trimester. Some public services offer the test for free. However, this depends on the hospitals in the area where the pregnant woman lives and the justification for having the test.
Health insurance plans, in most cases, only partially cover the exam, covering the preparation phase, consultations, and ultrasound, but not the aspiration procedure itself. This should be checked in each specific case.
What are the risks?
Although amniocentesis is considered a safe test, aspirating the fluid with a needle is an invasive method and always carries risks, even if minimal3. Statistics show that about 0.2% of women who undergo the exam are induced to miscarry afterwards, within a week. In much rarer cases, less than 0.01% of women, the exam can cause a serious infection. There is also a very rare risk, with a rate of 0.005%, of injury to the baby from the needle.
Another occurrence that may result from the amniotic fluid exam is a mild temporary loss of fluid through the vagina, which occurs in about 1.7% of cases. Usually, the loss of fluid stops within a week without causing further complications.
What are the warning signs after the exam?
In addition to the normal symptoms already described that may occur after the test, such as fluid loss, a little bleeding, and cramping for a few hours, it is recommended to seek immediate medical attention in the following cases:
- Heavier bleeding;
- Excessive fluid leakage through the vagina;
- Uterine contractions;
- Pain in the abdomen or back;
- Fever above 38ºC;
- Chills, malaise, or flu-like symptoms.
Amniocentesis results
The initial results, through molecular analysis, can be ready in 24 to up to 72 hours, but the complete analysis, called “fetal karyotype test,” can only be obtained within two weeks. The fluid sample needs to be sent to the lab, where the cells will be cultivated. That’s why it can take up to two weeks for enough cells to grow to give a reliable diagnosis.
The initial results, which relate to the most common chromosomal syndromes, can serve to reassure anxious parents if the test is negative. The baby’s sex is also revealed in these preliminary results. The best thing to do while waiting is to distract your mind and not imagine the worst.
See Also: Loss of Amniotic Fluid
Photo: BruceBlaus
