The authors present 2 unusual cases of haemoglobin (Hb) Bart's hydrops fetalis and highlight the problem of a screening system for α-thalassaemia which focuses on maternal and paternal mean corpuscular volume (MCV) alone. Normal paternal MCV may not preclude fetal Hb Bart's disease because of the rare occurrence of maternal uniparental disomy or non-paternity. During a mid-trimester anomaly scan, with fetal cardiomegaly or hydrops in a woman with low MCV but normal paternal MCV, obstetricians should remain alert for fetal Hb Bart's disease. This is very important and relevant for national screening systems in South-East Asia, where a routine mid-trimester scan may not be available. A routine mid-trimester anomaly scan should therefore be implemented and in high prevalence areas, sonographers should be sensitive to the cardio-thoracic ratio even if screening shows that pregnancy is unlikely to be at risk.