Amniocentesis or prenatal screening test?

When a woman is expecting a baby, it is a good idea to consult her doctor to decide as soon as possible on the most suitable prenatal diagnostic pathway, based on the patient’s age and other risk factors. In fact, there are many possibilities. You may choose to have a noninvasive test, such as the Bitest or the fetal DNA testing, or invasive prenatal diagnostic tests, such as amniocentesis or villocentesis.

Amniocentesis is a diagnostic test to detect with certainty the presence of possible chromosomal abnormalities in the fetus such as Trisomies 18, 21, 13. 1 It is often performed when the mother-to-be has already had a prenatal screening test that came back positive. Amniocentesis is a widely used invasive test that is performed by taking, through a puncture of the amniotic sac, a sample of amniotic fluid. 1

Amniocentesis, when it is done

Amniocentesis can be performed in the early stages of pregnancy, between 16 to 18 weeks, or it can be later, that is, performed after the 25th week of gestation.

It is an outpatient examination, and once performed, the patient is sent home where she must rest for 12-24 hours.

The amniotic fluid collected, is analyzed to detect any chromosomal alterations, numerical or structural, responsible for various genetic diseases such as Down syndrome, Edwards syndrome, Patau syndrome, Turner syndrome, Klinefelter syndrome and many other chromosomal abnormalities 2 . The waiting time for the result is about 15 days.

Examination at risk?

Performing this invasive test, carries risks because of the method of fluid collection itself. In fact, amniocentesis has an approximate 1% risk of miscarriage 3 . Therefore, the choice to perform it should be taken with much consideration, and usually the test is suggested to women with a high risk of chromosomal abnormalities, those who are familiar with some of these diseases, or women older than 35 years of age.

Amniocentesis is also recommended for those who have already had a noninvasive prenatal screening test and whose result showed a high risk index or even in cases where, by ultrasound, a possible abnormality is visualized. Same if the gestnate contracts any infectious diseases, such as rubella, toxoplasmosis, or cytomegalovirus 2 .

Results no earlier than the 19th week

In addition to the risk of miscarriage, another of the limitations of this procedure is that results cannot be obtained before the 19th week of pregnancy. Choosing to terminate a pregnancy at this gestational age brings with it very strong psychological consequences 3 . However, performing amniocentesis before 19 weeks brings difficulties in taking a sample because of the little amniotic fluid taken and the longer time frame. The risk rate of miscarriage with early amniocentesis rises 2.3% 3 .

Screening tests are noninvasive

Unlike invasive tests such as amniocentesis, noninvasive prenatal screening tests are safe and return the percentage of risk that a chromosomal abnormality is present in the fetus (which is why they are called “probabilistic”). The fetal DNA testing for example, is an early prenatal screening test that can be performed as early as 10 weeks gestation. It also has a high reliability rate of 99%.9%, in the detection of major chromosomal abnormalities such as Down syndrome and microdeletions.

It is always a good idea to consult your gynecologist to decide which prenatal diagnosis or screening pathway is best for yourself and your baby.