The definitive diagnosis in a newborn is by full karyotyping, although DNA-based techniques such as quantitative fluorescence PCR (QF-PCR) or fluorescent in-situ hybridisation (FISH) may be used simultaneously as an urgent initial way of detecting three copies of chromosome 18 material.
During pregnancy, QF-PCR and karyoptyping may be offered from a chorionic villus sample (CVS) or amniocentesis sample, where serum screen or maternal age indicate a pregnancy at increased risk, or where there are abnormalities on fetal ultrasound scan.