Twin foetuses. Coloured magnetic resonance imaging (MRI) scan of a sagittal section through twin foetuses in the womb. The twin at right has died in the womb, the other twin appears healthy. Non- identical (dizygotic) twins develop when two separate eggs are released from the ovaries and are fertilised simultaneously. Identical (monozygotic) twins occur when the two cells formed by the first division of a fertilised egg develop independently.