This study was aimed at testing if measurement of adult hemoglobin (HbA) by ion exchange high-performance liquid chromatography could serve as a purity control of fetal blood samples. We studied 240 samples obtained for karyotyping by cordocentesis under ultrasound guidance. Mean red cell volume (MCV), red cell distribution width (RDW) and HbA were measured on each sample. HbA was determined from 5 microliters of blood in 8 min. From 18 to 30 weeks of gestation, HbA values in fetal blood do not vary and are tightly clustered around 5.4% (SD:1.1%). After 30 weeks, HbA increases as the fetal to adult switch begins. Experimental contaminations of fetal blood by maternal blood show that HbA variations are more pronounced than MCV or RDW variations. A 5% contamination is readily detected. This approach is rapid, sensitive and reliable. Incidentally, it readily detects the presence of an abnormal hemoglobin.