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Embryo and yolk sac. Coloured 3-D ultrasound scan of a 30-day-old embryo (centre) and its yolk sac (left) attached to the wall of the uterus. The yolk sac provides nourishment for the embryo until the umbilical cord is fully functional. The yolk sac also produces blood cells, until the embryo's liver starts to make them at around seven weeks. 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. 3-D scanning uses computer technology to return more detailed images than conventional 2-D scans. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR NAJEEB LAYYOUS)