Foetus. Coloured 3-D ultrasound scan of a tightly tucked-up foetus at eighteen weeks. At this stage the foetus measures 14 centimetres crown to rump. The foetus' head is at top centre, one arm is held against the side of the head and a leg is at centre right. This is the time the mother will start to feel the baby kick, as it begins to flex its limbs. 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. 3-D scanning uses computer technology to return more detailed images than conventional 2-D scans.