Section 2(2)(a) in The U.P. Government Servants (Special Provisions Relating to Family Planning) Rules, 1976
(a)where one child or more than one child of the couple-(i)suffer from serious physical disability such as blindness, being crippled or mentally retarded so as to render it unlikely for him or her to lead a normal life; or(ii)suffer from such a disease that his or her long survival is doubtful, and the number of remaining children in normal health does not exceed two; or