Isabella being pregnant, effectively ended the Marlena/Roman/John/Izzy B quadrangle (although the Marlena/John/Roman triangle lived on for many many more years. Up until Isabella told John she was pregnant, he was still torn between the 2. And Marlena was equally torn, even knowing Roman was Roman, and John was John.
So even that pregnancy wasn't just a happy couple looking forward to a birth. It forced John & Marlena to move on, even though (as we now know) neither one truly wanted to. Yes, John loved Izzy B, and Marlena loved Roman, but I always felt that the pregnancy was written in to end the quadrangle - and not because John & Izzy B were happily married and trying to make a baby.