In my case it works in two ways:
As a horror story. Once I read a fanfiction in which a male character got impregnated agaisnt his will and as the unborn grew within him, the aspects of what defines a man as such began to play, while the horror of having no natural orifice to birth plus being forced to get performed on a complex surgery were things that deteriorated him emotionally.
After that first fic, while reading more, my perspective changed to a nurturing one. It has no relation to dominance or submisive traits to have a child when it comes to me, but the opportunity to create and bring life into this world, to be capable of doing some tiny miracle since it's quite difficult to get pregnant in real life for most (at least 18 months actively trying at least twice per week to reach more than 60% of chances).
The fact pregnancy brings both fear and high hopes with it, a vulnerable state that remains after birth, the realization things will change from then onwards, the raw power that exists within giving birth, it's interesting to me.
I also love the complex ideas authors have behind certain fics and doujins. They are quite creative.