When Holly Marie Combs became pregnant with her first son, the producers decided it was better to write the pregnancy into the show instead of trying to hide it. In fact, the writers did try to hide it at first, and it just didn’t really work. It was clear that there was a baby bump.
The problem was the pregnancy happened at the start of Charmed Season 6. At the end of Season 5, the character Chris had been introduced and it seemed there was a full back story behind him. He did some dastardly things throughout the Season 5 finale and the Season 6 premiere, making it very easy for people to believe that he was going to be the big bad of the season.
In fact, some of the things he did never made sense in the future. For example, he separated his parents, making it very possible that he wouldn’t be conceived.
But Combs was pregnant and the producers decided—what seems to be—somewhere in Season 6 that her character Piper would be pregnant too. This turned the mysterious person from the future into Piper’s second son, hell-bent on saving his older brother from evil. There was even a six-month time jump part way through the season to help fit in with Combs’ pregnancy, suggesting that it hadn’t quite been thought out fully.
While Charmed Season 6 did have one of the most-watched episodes after the reveal, there are still plenty of fans who wonder who Chris would have been had Combs not been pregnant.