Foetal twins. Coloured ultrasound scan of the pregnant womb of a woman, showing twins. The round heads of the foetuses are at upper frame, seen in top view. Their bodies are not clearly visible. 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 or fraternal twins result from two different eggs being fertilised. Ultrasound uses high-frequency sound waves to reveal structures inside the body, enabling the healthy development of the foetuses to be determined.