Hang on I gotta show this M×H baby I think about all the time real quick (https://www.tumblr.com/stardella/798671089428512768/cover-me-in-sunshine-by-somethingstrangest-had-me).
TL;DR, I think yes! Spam is a human ghost but he's awoken by Tenna's magic, so I could imagine a monster-baby, which is mostly magic, would work.

Rather than Spam being pregnant as a human and sneaking underground, the ghost route is much more canon-aligned (forcibly separated by war, Spam kept by humans from making it Underground and dying a grieving madman, brought back as a ghost the ■■■ years later when monsters are freed and Tenna finds his grave) and I love blook-fied ghostSpam. It kinda depends on "could I get away with ghost pregnancy?" and "if Tenna makes him his puppet body (which some folk on Tumblr hoped, I'm kinda eh on it), the magic in it could allow him to carry a baby, since monsters are made of magic."

Obviously I can make it happen. I can make it happen so easily.