Prenatal diagnosis is the process of testing fetus in terms of various diseases (chromosomal and single gene disorders) during pregnancy by selecting appropriate methods according to the week of gestation. For this purpose, most commonly used procedures are amniocentesis, chorionic villus biopsy and cordocentesis.

Various abnormalities of the baby and chromosomal disorders coexist at a very high rate. Severe growth retardation and oligohydramniosis, polyhydramniosis, NIHF (non-immune hydrops fetalis), Cystic Hygroma, Unilateral pleural effusion, Choroid Plexus Cyst, Hydrocephalus, Nuchal Edema, Omphalocele, Duodenum Atresia, Increased fetal abnormalities such as cardiac anomalies and obstructive uropathy are intrauterine irregularities. Fetal anomalies such as Cardiac anomaly and Obstructive Uropathy are intrauterine irregularities that lead to an increase in chromosomal abnormalities.

In the light of these findings, abnormalities detected by ultrasonography constitute another important reason that requires prenatal chromosomal analysis. High risk in screening test or detection of abnormality in ultrasonographic follow-up of pregnant women are among the leading indications for prenatal diagnosis.

Which pregnant women prenatal diagnosis should be applied to?

1. Advanced maternal age (over 35 years old)
2. Increased risk shown in double / triple screening
3. Increased risk shown in noninvasive prenatal diagnosis (NIPT) testing
4. Detection of abnormality of the baby in ultrasonographic examination
5. History of giving birth with chromosomal abnormality (for example, Down syndrome) from previous pregnancy
6. Balanced translocation carriage detected in either or both spouses
7. Molecular genetic examination for the relevant disease (thalassemia, hemophilia, sickle cell anemia, muscular dystrophy etc.) in the presence of a single gene disease in the family
8. Anxiety

What tests are performed at what week of gestation?

Chorionic villus sampling (CVS) performed between 10-13th week of pregnancy. It is applied in the 2nd or even the 3rd trimester when necessary, but this kind of sampling is more called placental biopsy. With the chorionic tissue samples taken, information about the baby is achieved by analyzing the chromosomes obtained from these tissues based on the principle that the tissues of the baby are of the same origin. Although it is mostly preferred for chromosome analysis, it is used in the diagnosis of both chromosome and single gene disorders.

Amniocentesis can be applied from 16th week (16-22 weeks) of pregnancy. Cells fell from the skin and mucous membranes of the fetus during fetal development accumulate in the amniotic fluid. Amniocentesis is a cytogenetic or molecular genetic test of fetal cells in amniotic fluid taken by entering the gestational sac with ultrasound-guided needle. It is the most widely used test in many clinics because it is reliable and has less risk of miscarriage than others.

Cordocentesis is performed by taking a blood sample from the umbilical cord under the guidance of ultrasonography, to make the chromosome analysis from this sample. The test can be applied from the 18-20th week of pregnancy, but the difficulty and risks of application are higher than the other two methods. Cordocentesis is generally applied in advanced pregnancy cases where previous tests (amniocentesis and CVS) failed or some abnormalities of the baby were detected by ultrasonography. Its important advantage is that it gives results in a shorter time (1 week) compared to the other two methods.