All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The postmortem does not nullify ablution or Ghusl. Hence, after conducting it, it suffices to wash the hands if they have touched impurity. It may also be said that Ghusl or ablution is recommended after postmortem, in view of the statement of the Prophet, sallallaahu ‘alayhi wa sallam, “Whoever washes the dead body of a person should perform Ghusl, and whoever carries it should perform ablution.” Indeed, postmortem makes one touch the dead body more than in the case of washing or carrying it.
On the other hand, postmortem is permissible in case of dire need or necessity. Shaykh Ibn ‘Uthaymeen said: “Conducting postmortem on the dead body of a Muslim person after his death is permissible only in case of dire need or necessity; and even if there is a need or necessity for that, the matter should be estimated in proportion to that need or necessity, after which the body should be restored to its original state.”
For further information, please refer to Fatwa 87495.
Allaah Knows best.