Complex sentence: Many also shave their head as this is considered hygienic, but the vast majority wait until after they have finished Umrah or Hajj to shave their heads, as this is a requirement to leave the state of Ihram.
Split into simpler sentences: Many also shave their head as this is considered hygienic. Most will wait to shave their heads until after they have finished Umrah or Hajj, as this is a requirement to leave the state of Ihram.