First-trimester screening is a noninvasive, risk-free procedure (ultrasound examination combined with maternal blood sample) which is conducted in the early part of pregnancy to assess the individual risk of the presence of chromosomal disorders (trisomy 13, 18, 21) in the fetus.
We recommend that the blood sample for first-trimester screening be taken several days before the ultrasound examination. The results are then available when the ultrasound examination is performed so that the findings can be conclusively discussed.
In calculating the risk, the factors taken into consideration are maternal age, two laboratory values (free ß-HCG and PAPP-A produced by the placenta) from the maternal blood, the thickness of the nuchal fold in the fetus (nuchal translucency, NT) measured on ultrasound and further ultrasound parameters (e.g., nasal bone, tricuspid valve blood flow).
On ultrasound examination, the extent of an accumulation of fluid in the fetal neck (nuchal translucency, NT) is measured. Minimal nuchal translucency is normal. As the thickness increases, so does the risk of chromosomal abnormalities and other diseases.
The test result is given as an individual probability for the presence of the most frequent chromosomal abnormalities (trisomy 13, 18, 21). If the risk is low (e.g., 1:2000), the test result is considered normal. In such cases, the low risk is usually accepted, and no invasive diagnostic test is undertaken (amniocentesis, chorionic villus sampling). A further reduction of the risk for the most frequent chromosomal abnormalities is possible by performing the non-invasive prenatal test ( see NIPT).
If the risk of chromosomal abnormalities is higher (e.g. 1:50), the test result is considered abnormal. The test results demonstrate an increased risk of chromosomal abnormalities in about 3%-5% of all first-trimester screening examinations conducted and in approximately 90%-97% of pregnancies with chromosomal abnormalities, depending on the quality and the extend of the ultrasound examination.
In cases of higher risk, parents usually want certainty about the presence or absence of a chromosomal abnormality. This is then provided by amniocentesis (examination of the amniotic fluid) or chorionic villus sampling (sampling the placental tissue).
First-trimester screening can only be performed between the 12th (11+) and 14th (13+) weeks of pregnancy since the accumulation of fluid behind fetal neck is a temporary effect which only exists between about weeks 10 and 15. Since increased nuchal translucency thickness can also be associated with other developmental problems (such as heart defects), it is advisable in such cases to undergo further ultrasound diagnosis of malformations (see below) in addition to chromosomal analysis.
During ultrasound examination in first-trimester screening, we also perform an early diagnosis of anomalies, which permits us to detect other developmental problems in the fetus. This early anomaly diagnosis does not, however, in any way constitute a satisfactory substitute for the more advanced ultrasound diagnosis of organic anomalies (e.g., heart defects, brain malformations), which is usually advisable between the 20th and 22nd weeks of pregnancy (high-resolution ultrasound, anomaly diagnosis, DEGUM II/III ultrasound).
Early diagnosis of malformation and measurement of nuchal translucency thickness require high-resolution ultrasound equipment as well and time and experience on the part of the examiner. The ultrasound and laboratory examinations are certified by the Fetal Medicine Foundation (FMF) London and the German FMF, and are annually re-examined. We hold certificates of competence for all currently available ultrasound measurements of the Fetal Medicine Foundation (nuchal transparency, nasal bone, tricuspid flow, ductus venosus flow).
At first trimester screening it is also possible to estimate the risk for the occurence of preeclampsia and placental insufficency. In case of an increased risk preventive treatments may improve pregnancy outcome.
In the further course of the pregnancy, the first-trimester screening can by supplemented by the so-called quadruple test, the non-invasive prenatal test (NIPT), and a high-resolution ultrasound scan in the 20th to 22nd week of pregnancy.