I don't know anything scientific about babies' weight gain, but it's really noticeable with Piglet that she'll go through a stage of putting on lots of fat, then suddenly shoot up in length/height and look almost slim for a while (except for the little thigh rolls!), then start laying on fat again, and so on. So if I'd been recording her weight and height on a regular basis presumably the two lines would look more like recurrently overlapping staircases (if you can visualise that?!?) than two nice smooth parallel curves...