Gad Zooks, Catherine looks ill in the pictures of her church visit with the Middletons.
That was probably as much as she could tolerate for one day, as she must still have her illness. If this Sandringham tradition occurred a month or two later, it could have been different. Do you realize that when a person has hyperemissus gravidus, she can unexpectedly throw up at any time, even after an hour or two being OK? It would have been easier to have quick access to the "bathroom" or "loo" at the church of the Middletons. Probably all set up in advance by her aides, with a security man next to the bathroom just in case she had to use it suddenly. When she really begins to recover, it would not be such a potential very bad photo opportunity for her to walk or ride to the church in Sandringham.
What I'm wondering now is, what if Kate has another baby; will she have to time it so she won't possibly be sick at Christmas?