Triplet pregnancy. Coloured 3-D ultrasound scan of a triplet pregnancy, of which only two foetuses remain (upper left & lower centre). A third amniotic sac (upper right) shows that a third foetus had been present, but was spontaneously lost. Non-identical triplets occur when three separate eggs (ova) are fertilized simultaneously. Triplets occur in around 1 in 8000 pregnancies. Ultrasound scanning is a diagnostic technique that sends high-frequency sound waves into the body via a transducer. The returning echoes are recorded and used to build an image of an internal structure. Foetal ultrasound scanning is routine during pregnancy.