Foetus sucking its thumb. Ultrasound scan of a full-term (9 month old) foetus seen sucking its thumb in the womb. The head of the foetus is visible (at left) in profile, its face facing upwards. Its hand is at centre right, with fingers curled into a fist. The thumb is being sucked. The chest of the foetus is at lower right. At full- term the foetus crown-rump length is about 36 centimetres, and it weighs some 3.4 kilograms. Its facial features are fully developed (as seen at birth). Foetal thumb-sucking is detected commonly in foetal scans. It may serve a similar function to thumb-sucking in young children, that is as a comforting and reassuring behaviour.