Combined first-trimester screening represents the gold standard of risk assessment for the presence of trisomy 21, 18, and 13. The concept is based on the age risk, the measurement of fetal nuchal translucency (NT), and the determination of serum markers free beta-hCG and PAPP-A in maternal blood.

In recent years it has been shown that the risk assessment can be improved by combining in-depth ultrasound and cell-free DNA analysis from maternal blood. In their latest study, the investigators were able to detect all fetuses with trisomy 21, 18, and 13 through this procedure. No normal fetus displayed an increased risk. In contrast, the detection rate in classic, combined first-trimester screening is about 95% and the false-positive rate is 3-5%. In this study the investigator examine the test quality - especially the false positives - of cell-free DNA analysis on trisomy 21, 18 and 13 as well as on the microdeletion 22q in 1000 pregnancies.

Inclusion Criteria:

• Maternal age of 18 years and more
• Crown rump length 45 - 84mm
• Referral for first trimester risk assessment
• Singleton pregnancy
• Written consent

Exclusion Criteria:

• No consent
• Known parental microdeletion 22q11.2
• Crown rump length <45mm or >84mm
• Multiple pregnancies including vanishing twins

Pregnant women who are referred for FTS or for further follow-up examinations in case of a suspected anomaly or increased nuchal translucency at 11-13 weeks' gestation