Twin foetuses. Coloured ultrasound scan of twin foetuses at 12 weeks. The foetus' heads are at bottom right, with their bodies extending to centre. Non-identical (dizygotic) twins develop when two separate eggs are released from the ovaries and are fertilised simultaneously. Identical (monozygotic) twins occur when two cells formed by the first division of a fertilised 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.
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