In portuguese language, ALL the words carry the stress on one of the last three syllables. If it's on the last syllable, the word is called "oxítona". If it's on the second from last, the word is called "paroxítona". And if it's on the third from last, the word is called "proparoxítona". ALL the "proparoxítonas" have accent. Pe-rí-o-do fits that rule.