Okay, but considering that we don't know how demon pregnancy works it is still a possibility, even if it is a infinitessimally small one.
(I'm saying that because of that one WN setting that comes to my mind when I think about demons and pregnancy. I forgot the title, but the gist of it was that the protagonist had a demoness as mother and went out to gather the ultimative harem, like no other protagonist ever did before. It was apparently so harem-y, that even the protagonist's own mother decided to sleep with him or at least become a member of his harem. As a result, the author had to find a way to avoid the incest issue and continued to bend him-/herself into a pretzel, spinning a yarn about how the demoness found the protagonist as a small child/baby who was about to die and decided to take him into her body, thus re-borning him as her son-yet-not-son. She basically un-birthed the human baby and carried him to term. And now they're mother and son, yet not, just to ensure that nothing stands between the protagonist and his newest, bonable harem member. Except maybe a boner...)
PS: I never read the WN in question, only checked a discussion forum for it. It's also been quite some time since then, so my memories of the series may be inaccurate. Anyway, my point was that if the author wants to, a pregnant demon can always not be what it appears to be...