It could be a memory of a dream of a memory of a dream.. Just kidding.. well it could be.

But I'm not here to doubt peoples' memories.. It's more like piecemeal. If you were a child having this dream, your brain is developed enough to do logical thinking about things not present. So, in all reality you probably did have a nightmare about going through the birth canal.. A child can, under the right influence and stimuli, imagine what it may like to be born. A physical memory of confinement may be stored in the neonate's primal brain.. and an overwhelming anxiety over a void? nothingness? lack of safety? may have produced a nightmare about going through the birth canal. It sounds terrifying.

Overall, the things we conceive as long-term memories is not an actual function in a neonate. We have an infinite amount of influences to our stimuli, abstract ideas, etc. that our brain processes and puts together in ways we can understand logically all through-out the span of the developing brain (the myelinating and demyelinating pruning goes well into our 20s).

And I agree with whoever said it's not possible at this moment in science to understand everything, but what we do have scientifically is we are born with a developed primal brain and the functions of that primal brain. From there, what our developing brains do with those primal memories and how it processes it (such as body memories, etc.) has all the validity in the world.