I remember that Lexus was about a week to ten days away from delivering when the kittens were at their most active. But, all kitties are different, so your mileage may vary.
In my experience, and maybe it is just that we are not that observant, about the time the cat looks like she will explode... is about the time she has kittens. at least within a week. Albeit I have only seen 3(that I know of) pregnant cats and all the times when they looked that big, the kittens were on the way in not very long. But what may be constant for me, may not be constant for everyone.