An early test that combines information from ultrasound examinations and biochemist and evaluates the level of risk to the fetus of Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome).
These Trisomies, so designated because a trio of chromosomes replaces the usual chromosome pair 21, 18 or 13, are characterized by degrees, more or less pronounced, mental retardation and various malformations that can affect different organs and systems, from the Central Nervous System to the kidneys and heart, which may be concerned the own survival.
The risk of developing these Trisomies in the fetus increases with maternal age, especially from the 35 years, but often find young women with children suffering from Down syndrome. The maternal age, the ultrasound examination and biochemical examination, allow a detection rate of about 90 to 97%. The prenatal Screening must be performed between the 11th and the 13th week + 6 days of gestation, after the date of the last menstruation.
The ultrasound exam will make it possible to determine with accuracy the gestational age, the Skull-Length (CCC), the subcutaneous space located on the neck of the fetus (neck Translucency-TN) and assess the presence of Nasal bone (ON). In biochemical examination there is the determination of two substances present in maternal blood-Plasma protein the associated to pregnancy (PAPP-A) and ß Subunit free Gonadotrofica Chorionic hormone (Beta-HCG)-both with great sensitivity and specificity for the screening of anomalies.
The result of screening indicates the risk that the fetus has been born with chromosomal abnormalities and is expressed as: (positive screening) or Below (negative screening). A positive screening, means that the fetus has a high likelihood of having a (there is 3% false positives, i.e., positive diagnosis and absence of anomaly). A negative screening reveals a low probability of the fetus present malformations, but does not ensure a regular baby when wearing maternity underwear.
The individual risk of a pregnancy can be evaluated using the combined first trimester Screening, thus allowing parents, with the attending physician, make an informed decision in favor or not of the invasive exam. The definitive diagnosis of malformations referred to can only be determined through invasive examinations, as the villus biopsy done or amniocentesis.