My mom in law hv vitiligo husband don't have.as i know it's heriditory.wat precaution can be taken before pregnancy to avoid it completly?

No but: vitiligo is considered mostly an autoimmune disorder but per se is not hereditary although the tendency of developing autoimmune disease may be. There is no known way to prevent its development other than treating the autoimmune disorder when and if it occurs. Even then, there is no way to tell whether the vitiligo would subside or resolve. I'd not lose sleep over this.