Phenylketonuria is the best known pathology of amino acid metabolism. Presented here is the case of a 23-year-old prima gravida with phenylketonuria since birth. After delivery, her child was diagnosed with free trisomy 21. Abnormal sonographic signs such as bilateral hydrothorax, polyhydramnion, and short femura under the 10th percentile could be demonstrated in the ultrasound scan at 33 weeks of gestation. Regularly measured maternal phenylalanine levels during the complete pregnancy as well as preconceptionally were always under the embryopathic cutoff point of 1 200 micromoles/L (20 mg/L). An association seems unlikely. This is the first description of such a constellation according to a literature search (PubMed, Cochrane Library).