I know, tens of "cum_*" tags, but if the discussions on various images are right, cumming right into the uterus should be a physical impossibility. However, this is seen in hentai all the time. Because of its uniqueness to the illustrative medium(s), I feel cum_in_uterus would be a worth-its-existence tag. (I would also vouch for penis_in_uterus, but I don't know if that's common enough.)
Updated by Shinjidude