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Twin foetuses. Coloured 3-D ultrasound scan of twin foetuses. Non-identical (dizygotic) twins develop when two separate eggs are released from the ovaries and are fertilized. Identical (monozygotic) twins occur when two cells formed by the first division of a fertilized egg develop independently. 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. 3-D scanning uses computer technology to return more detailed images than conventional 2-D scans. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR NAJEEB LAYYOUS)