We do sometimes. DK photographs everything for their books and since we have a lot of pregnancy books, we have a huge archive of material to use as needed. But we do have to update those pictures - it's a bit hard to sell a new pregnancy book when the woman has a 1980s haircut! If we do any sort of specialist topic, i.e. twins, we may not have enough images to work with so we have to do photoshoots.
I will say that in order to save money we're encouraged to re-use images we already have; I've been at DK for over a year now and only seen calls for models about 5 times. If anything comes up again, I will make an announcement here!