WHAT IS THE REASON FOR BIRTH DEFECTS?

It is due to the prArabdha karma phala that one is born with deformity. Such a birth unfortunately one has to take in order to experience the sufferings.

According to Manu Smriti, one who does the karmas with whatever bhAvas, with the same type of SarIra, he experiences the fruits of his action .

yAdruSena hi bhAvena yadyatkarma nishevate!
tAdruSena SarIreNa tattatphalamupAsnute!!
--- Manu smruti

According to Sankha, one who kills a BrAhmaNa is born as a kushtha rogI, one who kills cow is born blind.

brahmahA kushthI ! goghnascAndhah !
--- Sankha

According to yAjnavalkya, one who steals annam gets afflicted with indigestion. one who steals oil is born as a cockroach.

annahartA AmayAvI syAt ! tailahrut tailapAyI syAt !
---- yAjnavalkya