Scan of a foetus (21 weeks). Coloured ultrasound scan of a profile of the head and body of a human foetus aged 21 weeks, in the womb. At top is the rounded cranium, with the foetal face seen at upper right. The face appears completely human with fully developed nose, lips, chin and external right ear. At centre and lower centre is the body, with an outline of the spine and an upper arm seen. The crown-rump length (sitting height) of the foetus is about 20cm and its weight some 500 grams. The head of a foetus at this age is proportionately large compared to its body. Ultrasound makes use of high-frequency sound waves to obtain slice" images through the body."