Twin foetuses. Coloured abdominal X-ray of a pregnant woman showing her healthy twin foetuses. They are 30-32 weeks old. The bones of the foetuses are pink, while their mother's bones are yellow, blue and pink. One foetus is lying horizontally and face down with its skull at upper right, while the other is vertical with its skull in its mother's pelvic cavity at lower centre. Non-identical (dizygotic) twins develop when two separate eggs are released from the ovaries and fertilised. Identical (monozygotic) twins occur when the two cells formed by the first division of a fertilised egg develop independently.