Twin foetuses. Ultrasound scan of twin foetuses in the first trimester of pregnancy. Non-identical (dizygotic) twins develop when two separate eggs are released from the ovaries and are fertilized simultaneously. 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.