When the fiancée of a U.S. citizen is pregnant with their child and waiting for a K1 visa, this doesn’t impact the visa’s timeline or process. Having a child together could help in terms of evidence that your relationship is bona fide, but it will not expedite the process. The child in question could be eligible for US citizenship, but there are conditions that need to be met and you should make sure you are aware of the requirements of submitting a Consular Report of Birth Abroad. There is the possibility that the U.S. Embassy might also request a DNA test as part of this process. However, if your child is eligible for U.S. citizenship, you will not have to file for a K2 visa as the child is already a citizen.
Typically, consular officers also request a sworn affidavit from the U.S. sponsor that recognizes the pregnancy. It’s a good idea to have this sworn statement from the U.S. petitioner in case it is requested of you during your interview.