JJ's story in Criminal Minds?

Okay, so, i saw an episode of Criminal Minds of Sky Living two weeks ago, and i instantly fell in love with Reid and i liked JJ as well. Then, i watched an episode the weekend after, and JJ wasn't in it. I was asking my friend (Who has seen every single episode) what happened to JJ and she said that she was... show more Okay, so, i saw an episode of Criminal Minds of Sky Living two weeks ago, and i instantly fell in love with Reid and i liked JJ as well. Then, i watched an episode the weekend after, and JJ wasn't in it. I was asking my friend (Who has seen every single episode) what happened to JJ and she said that she was pregnant in the show and left 'to go and have the baby'. My friend said that this was at the beginning of Series 4. I then watched an episode today that i'd recorded, and JJ was in it again, and that episode was in Series 5. i then watched a series 6 episode, and JJ failed to star in that. Can you please tell me which of this information is true or tell me what actually happens to JJ?
So:
Series 4: JJ leaves
Series 5: JJ comes back
Series 6: JJ's gone again

Thanks for your help!
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