Post Conception Weeks (PCW) 
Foot Length (mm) 
Knee to Heel Length (mm) 
Gestational Age in Weeks 

Above 5 and less than 6 
8 
10  
6 or above and less than 8 
11 
11  
8 or above and less than 10 
13 
12  
10 or above and less than 13 
17 
13 

12 
13 or above and less than 17 
24 
14 
17 or above and less than 20 
31 
15  
14 
20 or above and less than 23 
36 
16 
15 
23 or above and less than 25

40 
17 
16 
25 or above and less than 27 
43 
18 
17 
27 or above and less than 31

51 
19 
18 
31 or above and less than 33 
56 
20 
19 
33 or above and less than 36

60 
21 
20 
36 or above and less than 40 
66 
22 
W.M. Hern. Correlation of Fetal Age and Measurements between 10 and 26 weeks of Gestation, Obstetrics and Gynecology. Vol 63, No1, 2632, January 1984. 