First off I'd like to say how much I admire you deciding to have contraption as part of your plan with your partner. I say 'admire' becuse it is flying in the face of all your instincts, even if a sanction of you mind says it is ostensible for your partner, the child and you. No easy thing.
To be constantly 'nagged' by unwanted feelings is not a good way to
live, and your distress can even spill over to affect those you love.
As you already have an idea that your mind because of OCD does not always behave as it might, may I ask if you are having medical support? I found my conditions simply got worse until I had competent medical help, after which in time things turned around.
If you have support already, may I suggest that reviewing it so as to make life easier might be a good idea.
I'd imagine there have been other compulsions or beliefs in the past, do you remember what were the things that enabled you to overcome them?
You have already said you have discussed having a child with your partner. To have someone that can lend perspective is one of the things I've found truly valuable (apart from love and care of course)
I would like it if you could talk some more