Quadruplet foetuses. Coloured 3-D ultrasound scan of quadruplet foetuses. One foetus is clearly seen at centre right. The other three are arranged from top to bottom down left. Quadruplets can result from the simultaneous fertilization of one, two, three or four eggs. Quadruplets occur naturally in 1 in 700,000 pregnancies, but are more common in women who have received fertility treatment, as here. 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. Foetal ultrasound scanning is routine during pregnancy.