Abstract
Maternal serum screening for Down's syndrome and trisomy 18 identifies pregnancies with a greater risk of these abnormalities, which are then followed-up by karyotyping of cells collected either by amniocentesis or by chorionic villus sampling. These techniques complement ultrasonography, which gives accurate gestational dating as well as identifying structural abnormalities. Other chromosomal abnormalities are sometimes detected by virtue of atypical maternal screening results. This report illustrates a case of triploidy, a lethal abnormality, detected incidentally due to an exceptionally high human chorionic gonadotrophin result identified during Down's syndrome screening. This allowed appropriate counselling of the parents followed by a decision to terminate the pregnancy, avoiding the potential trauma of a spontaneous miscarriage or, if born live, death of the baby. Termination of the pregnancy also resolved associated maternal hyperthyroidism.