Legitimacy has to be determined in line with Islamic Principles. Paternity is established by marriage between the parents of child. Under Sunni Law a child born after six months from date of marriage and within two years of termination of marriage is presumed to be legitimate child. In Shiah Law there is a little variation in respect of the period. The starting point of recounting six months is the date on which marriage is consummated and other limits is of ten months from dissolution of Marriage.
In every case where legitimacy of child is questioned the primary evidence is of mother of child. Who is the best person to testify that the child is legitimate issue from her husband. Whenever such evidence is proved to be disinterested independent and impartial it is always believed by court being truthful trustworthy and confidence inspiring. The discharge of negative onus is placed on the father.
Testimony of mother, being positive evidence obviously leases to be a direct inference of relationship of child with his father. Conversely father who denies his relationship with child is hardly able to establish so as his stand being negative cannot be easily proved. Onus placed on child or his mother to prove legitimacy can be discharged conveniently and positively than discharge of a negative complaced on father.