First trimester screening

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The first trimester screening ( ETS ; in Switzerland first trimester test , ETT ; English First Trimester Screening , abbreviated FTS ) is a screening examination as part of prenatal diagnosis in the first trimester of pregnancy (the first trimester is the first 3 months of the 9-month pregnancy) to find of down syndrome in the unborn child.

The first trimester screening is one of three non-invasive methods of Down syndrome detection and, according to a test by Stiftung Warentest in 2009, ranks third in terms of security of evidence (i.e. the highest possible detection rate and as few false positive results as possible) behind the integrated screening and the Sequential screening .

method

In the FTS, two biochemical laboratory values ​​from the maternal serum ( PAPP-A and free β-hCG ) are analyzed and the fetal neck transparency is measured using ultrasound . The combination of these results is used in a statistical comparison with a large number of known combinations of pregnancies with positive and negative diagnoses to determine the probability of the possible presence of a fetal chromosomal aberration , especially trisomy 21 (Down syndrome). These probabilities in turn modify the probability of trisomy 21 already existing in every pregnant woman (usually the so-called age risk or the risk of recurrence after a previous pregnancy with fetal trisomy 21).

An example: A couple has a first trimester screening carried out. The result provides a probability value of 1: 300 that the child could develop trisomy 21 . Based on the test results obtained, the genetic history of the parents, the age of the mother and the confidence in the test procedure (keywords True Positive Rate and False Positive Rate ), the parents - possibly in combination with the attending physician - can make a decision about further measures .

An extended first trimester screening (ETS) can detect up to 95% of the fetuses affected by trisomy 21 (Down syndrome). In 3 to 10 percent of fetuses without a trisomy 21, the ETS incorrectly determines an increased risk.

If the probability determined by first trimester screening exceeds a certain limit value, it has been customary up to now for the treating gynecologist to offer his patient a placenta puncture or an amniotic fluid test. Both methods are invasive interventions through which child cells are obtained for the actual diagnostic examination, the prenatal chromosome analysis. Only such an examination finally leads to an almost 100% confirmation or to the almost certain exclusion of a child's chromosome aberration. However, the invasive examinations lead to a miscarriage in 0.3 to 1% of the procedures (as of 2014). In Switzerland, a non-invasive prenatal test is usually offered first; if the calculated risk is greater than 1: 1000 as part of the first trimester test (neck transparency, blood test), the costs have been covered by the basic insurance since 2015 .

Investigation period

The first trimester screening is carried out at the end of the first trimester ( first trimester / first trimester ) of pregnancy , in a period between 11 weeks +0 days up to and including 13 weeks +6 days. New studies show that the informative value of this screening decreases from 11 to 13 weeks of pregnancy. The PAPP-A is significantly more informative, especially at 10 weeks and earlier (8–9 weeks) than at 13 weeks. Conversely, the free β-hCG at 11 weeks has a lower informative value, which only increases in the following weeks of pregnancy, with an optimum in the 2nd trimester (from 14 weeks). This discrepancy is avoided by the integrated screening , in which the risk markers can be analyzed in addition to the Doppler examination of the ductus venosus and the tricuspid valve .

Practical use

Based on different statistical algorithms and different test groups, there are several computer programs with which a first trimester screening can be carried out. It is important for the pregnant woman to have an estimate of the reliability of the method used. At the same time, the treating physician needs legal and ethical assurance that he is selecting a valid, i.e. demonstrably reliable, algorithm for the patient. He must therefore check to what extent studies verify the targeted first trimester screening and how high its error rate is. Since none of the algorithms commonly used today is freely available, it practically amounts to comparing different products from different providers. In the February 2009 issue of Stiftung Warentest, various of these medical products, which include such computer programs, were assessed. Since 2013, the genetic diagnostics commission at the Robert Koch Institute has called in its guideline for the requirements for carrying out prenatal risk assessment "the publication of the entire algorithm with detailed software specification that precisely and comprehensibly describes how the risks are calculated". In addition to the medical aspects mentioned above, productive use in laboratories also means that, depending on the provider and the quality of the screening, there are sometimes considerable initial costs.

Web links

Individual evidence

  1. ↑ a b Pregnancy: Down-Tests with Risks . In: Test . (Stiftung Warentest) Issue 2/2009, p. 84ff.
  2. ↑ a b Angelika Dohr, Vera Bramkamp: Non-invasive prenatal tests NIPT. (PDF; 159 kB) In: pro familia medicine. No. May 2 , 2014, accessed August 15, 2016 .
  3. ↑ Swissmom: Health insurance reimburses non-invasive trisomy blood tests
  4. ↑ Genetic Diagnostics Commission: Guideline of the GendiagnostikKommission (GEKO) for the requirements for the implementation of the prenatal risk assessment as well as for the necessary quality assurance measures according to § 23 Abs. 2 Nr. 5 GenDG. (PDF; 284 kB) April 2013, accessed on June 18, 2018 .