There is no Necessity For a Medical Examination Before Marriage
Question :
I want to marry my paternal uncle's daughter, but she and some of my close companions advised me to have a medical examination so that we may rest assured regarding the genes of heredity. Does this involve any interference in Allah's Decree and His Divine Predestination? And what is the ruling of the religion on this examination, may Allah grant you success?
Answer:
There is no need for this examination and you should both have a good opinion of Allah, for Allah, Most Glorified says, in the narration attributed to Him by the Prophet :
"I am according to how My servant perceives Me."
And also because the examination might give an untrue result. May Allah free us and you from every evil.
Source:
Ibn Baz
Fatawa Islamiyah, Vol. 5 Page 172