Miss. Code § 91-5-3
A devise so made, or any clause thereof, shall not be revocable but by the testator or testatrix destroying, canceling, or obliterating the same, or causing it to be done in his or her presence, or by subsequent will, codicil, or declaration, in writing, made and executed. Every last will and testament made when the testator or testatrix had no child living, wherein any child he or she might have is not provided for or not mentioned, if at the time of his or her death he or she have a child, or if the testator leave his wife enceinte of a child who shall be born, shall have no effect during the life of any such after-born child and shall be void unless the child die without having been married, or without leaving issue capable of inheriting, and before he or she shall have attained twenty-one years. The estate, both real and personal, so devised shall descend to such child in the same manner as if the testator or testatrix had died intestate, subject, nevertheless, to the bequests made in the last will and testament in case of the death of such child before marriage, or without issue capable of inheriting, and under the age of twenty-one years. When a testator shall leave children born and his wife enceinte, the posthumous child or children, if unprovided for by settlement and neither provided for nor disinherited, but only pretermitted, by the last will and testament, shall succeed to the same portion of the father's estate as such child or children would have been entitled to if the father had died intestate, towards raising which portion the devisees and legatees shall contribute proportionably out of the parts devised and bequeathed to them by the same will and testament.
Miss. Code § 91-5-3