DNA Test

My partner and I have been together since 2013, in 2014 he had a bone marrow transplant we weren't trying for a baby but it did happen and it's the best thing that has ever happened to me (besides my partner & getting engaged) anyways so he had to fly to see his doctors for the 2yr check up and so he tells me that the doctors want to get a DNA test because there has only been a few people that had bone marrow transplants that have been able to have children. I know it's for science and to try help other people but I feel so hurt. I have never cheated on my partner and this baby is defently his.