Die Bedeutung des Schwangerschaftsalters bei der individuellen Risikoberechnung für ein fetales Down-Syndrom in der sogenannten Triple-Diagnostik

Purpose: Individual risk assessment for fetal Down syndrome by use of serum markers (known as triple screening) involves several systematical errors which may significantly reduce the specificity of risk calculation. Concentrations of serum markers as human choriongonadotropin (hCG), unconjugated estriol (uE 3 ), and α-fetoprotein (AFP) are significantly correlated with the gestational age. Is it possible to minimise misjudgement of the gestational age as one of the most common and important sources of error in risk calculation? Method and Materials: The present study includes 11207 cases of routine triple-screen examinations where gynaecologists reported in combination: the individual data of crown-rump length (CRL) - 1st trimester, biparietal diameter (BPD) - 1st/2nd trimester, and gestational age derived from the doctor's specific biometrical criteria and tables. All these data were converted to gestational days using standardised tables and were extrapolated to the day of blood sampling. The resulting distributions of differences in gestational age estimation were compared to each other. Results: Comparisons revealed that the gestational age reported by the gynaecologists was overestimated in 73.4% of cases as to the age derived from BPD, and in 63.2% of cases as to the age derived from CRL. Comparing the gestational age derived from CRL (1st trimester) and BPD (2nd trimester) only 55% of cases were overestimated and 34.7% of cases were underestimated by BPD. Conclusion: The systematical increase of calculated risk caused by overestimation of the gestational age results in a surplus of indicated amniocenteses which may by avoided by the use of specific sonographical parameters (CRL -1st trimester, BPD - 2nd trimester) and by the use of standardised tables.