Objective To assess the relation between serum parameters and nuchal translucency in pregnancies affected by fetal aneuploidy in the first trimester. Design Retrospective study of different serum parameters collected prior to chorionic villus sampling and measurement of nuchal translucency in relation to fetal aneuploidy. Setting Switzerland (German and Italian sector) and Bregenz, Austria. Population One thousand one hundred and fifty‐one women aged 25 to 44 years at 10 to 13 weeks of gestation undergoing chorionic villus sampling, mostly for advanced maternal age. Fetal aneuploidy was found in 23 pregnancies including four cases of trisomy 21, five of trisomy 18 and one case of trisomy 13. Main outcome measure Fetal karyotype, serum levels of free β‐hCG, pregnancy‐associated plasma protein A (PAPP‐A) and alpha‐fetoprotein and the measurement of nuchal translucency. Results Serum PAPP‐A was decreased in all common chromosomal abnormalities. Free β‐hCG levels were increased in trisomy 21 but decreased in trisomy 18, whereas alpha‐fetoprotein was low in trisomy 21, 18 and other chromosomal abnormalities. Nine of twenty‐three abnormal embryos had evidence of an increased nuchal translucency. Nuchal translucency, however, did not seem to be associated with any alteration in the levels of the biochemical parameters in either chromosomally normal or abnormal embryos. A low serum PAPP‐A or an increased nuchal translucency was seen in two‐thirds of all pregnancies with chromosomal abnormalities. Conclusion A nuchal translucency 3 mm and depressed serum PAPP‐A levels have a good predictive value in the detection of fetal aneuploidy at 10 to 13 weeks of pregnancy. Serum free 0‐hCG and alpha‐fetoprotein levels may give additional information. An increased nuchal translucency was not‐associated with altered serum parameters. This would allow these different markers to be used in combination.