Twin foetuses. Ultrasound scan of twin foetuses aged around 20 weeks. Their heads are at centre left and right. Non-identical (dizygotic) twins develop when two separate eggs are released from ovaries and fertilised. Identical (monozygotic) twins occur when the two cells formed by the first division of a fertilised egg develop independently. In ultrasound scanning, high- frequency sound waves are emitted into the body via a transducer. The returning echoes are recorded and used to build an image of internal structures.