Triplet foetuses. Coloured 3-D ultrasound scan of triplets. Non-identical triplets develop when three separate eggs are released from the ovaries and are fertilised simultaneously. Identical triplets occur when cells formed by the first divisions of a fertilised egg develop independently. Triplets normally occur in about 1 in 8000 pregnancies, but are more common in women with a family history of triplets and women receiving fertility treatment. 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.