This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cornea, Subject name: Ophthalmology
Microcornea is defined as: 
Select and explain. 
A. < 10 mm corneal diameter at bih, B. <13 mm corneal diameter at bih, C. < 10 mm axial length at bih, D. < 20 mm axial length at bih
A: < 10 mm corneal diameter at bih
Answer: A: < 10 mm corneal diameter at bih 
Explanation: Microcornea < 10 mm corneal diameter at bih. Megalocornea > 13 mm after 2 yr of age or >11.7 mm before 2 yr of age.