Foetal twins. Coloured ultrasound scan of the pregnant womb of a woman, showing twins. The round heads of the foetuses are at upper frame. The foetus at left is seen in side view, its body below the profile of its head. At upper right is its twin with the head facing down and the body horizontal. Each twin is contained in its own amniotic cavity (black). Twins occur naturally in 1 in 80 pregnancies. Identical twins arise from a single ovum (egg) that splits after fertilisation; non-identical twins result from two different eggs being fertilised. Ultrasound uses high-frequency sound waves to reveal structures inside the body.