Ultrasound scan of a human foetus at 12 weeks showing head (centre right), abdomen (lower left) and legs (centre left). At this stage in its development the foetus is around 8 centimetres long and weighs around 18 grams. The face has all the human characteristics but the ears are still primitive. The sex may also be determined at this stage using ultrasound examination. This technique involves the passing of high frequency sounds through the mother's abdomen using a transducer. The echoes are recorded to build up a picture of the foetus. It is a useful way to monitor and examine the health and growth of the foetus.