When they ultrasound Phoebe's baby she is supposedly 8 weeks along. The image of the baby is clearly one in the second trimester.
After Paige and Phoebe summon Piper to the cage they're trapped in, they watch as The Seer is made the new Source of All Evil. When Piper says the line: "Are you sure there's nothing stronger than the Power of Three?" her lips don't move.