I've read a lot of Japanese hentai manga (porn comics) in which the cervix is penetrated as if it's nothing. At least when it's a tentacle monster doing it, the length is there. When it's a supposedly normal guy doing it, it shows the author is ignorant of penis to vagina proprotions or just doesn't care.
In a tentacle demon story I wrote, I made a specific point of saying the demon doesn't enter the cervix. However, I'm totally aware this fetish exists, and the imagery of the girl's abdomen protruding from the cock or tentacle pushing against the inside of the womb is a strong one, if only because it is so goddamn absurd. So in the interests of giving the audience what at least some of them apparently want, I've put some thought into this.
So as long as you're in a fantasy or scifi setting to begin with, there are two "plausible" things to remember, which has already been mentioned. One, the cervix opens fairly wide during childbirth. This is generally a function of hormones, all the inner workings of reproductive systems are regulated by hormones. Introduction of the right hormones at an inappropriate time could have interesting effects. Two, during normal periods the cervix is closed but its opening still runs through it. It's just very small. Something, or rather a lot of something, that is similarly very small should be able to get through unnoticed. Once on the other side, in the womb, they can have a little more fun. It's not quite the same as a full blown cervix penetration, but the end effect should be just as good to fulfill the fetish.