Just because it's planned doesn't mean that it hasn't become urgent. Actually on this thread we have commented that Kate has looked very tired in recent months so perhaps it is a condition that has worsened and caused increasing pain.
I'm really glad that William has also chosen to scale back his engagements to support his wife and the children rather than the stiff "duty first approach". I always thought it was terrible that CPV was forced to go on an an overseas trip to Greenland (with Fred and Haakon) while her new fiancé, Daniel, underwent his kidney transplant. The official statement was that it was a "family decision" but I always read this as pressure from the Palace and especially the King that she continue her duty.