This is true. Although I don't remember all the biochemistry from hygiene school, and I'm sure dental students get a much more in-depth explaination, most of the patients I have that are pregnant have only a very small difference in their gum tissue, if any.
Their gums might be a bit more spongy and you might see a little more bleeding, but for the most part that's it. I've never seen any patient with a pyogenic granuloma, or anything that even resembles that. Most women might notice a little more bleeding on flossing or scaling, but nothing dramatic. The pictures of all the stuff in oral path are a lot more exciting than what I see on a regular basis.
Good homecare and regular cleanings are important before pregnancy and throughout breastfeeding.