Foetus. Coloured ultrasound scan of a healthy 16- week-old (4-month-old) human foetus (yellow & purple) showing its curved spine (yellow, upper right). Its head is at centre left. At this age the foetus appears completely human. Fingers and toes will have formed and the skin will be covered with a fine hair (lanugo), while eyebrows and hair on the head develop. The crown-rump (sitting) length at 16 weeks is around 14 cm and the weight is about 200 grams. Ultrasound scanning uses high- frequency sound waves to visualise internal body structures. As well as enabling the foetus' development to be assessed, ultrasound is used to estimate the date of delivery.