Age of foetus is calculated by 'Rule of Haase'. The length of the foetus is measured from the crown to the heel in centimetres. During the first five months of pregnancy the square root of the length gives the approximate age of the foetus in months. During the last five months, the length in cm divided by five gives the age in months. Here, the length of foetus is 4 cm. Hence the age will be 2 months (8 weeks). Ref: The Essentials of Forensic Medicine and Toxicology by K S Reddy, 27th edition, Page 74-75.