Unfortunately the answer to your question is that it depends. Just like some women develop stretch marks on their belly and some do not, the changes that occur due to pregnancy affect all women differently. Your breasts will enlarge with pregnancy and remain enlarged in the postpartum period just as if you did not have a reduction. The fact that you do not breast feed will not necessarily prevent you from developing changes in your breasts after your reduction. I have often heard the comment, "my breast grew back again after my reduction". When you undergo a reduction, the breast tissue and fat is gone forever. However, the remaining breast tissue and fat cells can become enlarged due to weight gain and pregnancy. It is also very possible that your breast return to the same size, shape, and tone depending on your age.
If you already had a breast reduction, I would not recommend avoiding the beautiful experience of having a child simply because you are afraid your breasts will change. If you are planning on having a reduction and plan on having a child soon, I would recommend waiting until after the birth and after your weight has stabilized. Of course, it is important to see a qualified, experienced Plastic Surgeon to discuss your individual case.