Can we find mentally retardness in baby.

I have syster who is mentally challenged since her birth. She is physically normal but her wisdom level is not at par with her age. Now her age is 28 years but she is having wisdom of 5 years kid. My father and my grand father is lightly Innocents not mentally retarded. While my mother is pregnant when my syster is there in her womb, she had did all agricultural works and carried regularly heavy weights. Now my question is my syster was born as mentally challenged because my mother did all hard works or because of my father grand father inheritance ? I am recently married, now my wife is 4 months pregnant and I am taking care of her well and she is not doing any hard works. Now I am afraid whether my grand father, father, sister's inheritance will continue? "Is there any test or scan particularly that identifies this mentally retardness in mother's womb itself".