This is usually the woman's fear towards the end (of an exhausting pregnancy as the unknown draws to an inevitable and painful close - well if you live in Europe it tends to be such if you're not keen on epidurals). But you already informed us you were neurotic. (Love how involved you are though!)

My interpretation (alongside the nighmarishly Siamese one) is that, unmanifest fully to you yet, you can only imagine the infant as it originally was (two halves). How can it come out as one new being!? It is outside your control: hence worrying.

This is your mind overruling a more holistic sentiment that is harder to arrive at from a distance (it requires surrender to the organic process - there is only so much you as father can ever do that).

More Cabalistically, you recognise the Adam Kadmon in your child intuitively but know your psyche will want to think in terms of genes, (whose flaws or talents to blame or praise: t would be easier to determine that if he kept them nicely split down the middle). All that falls away once he's his own person, but when he's a teen you may wish to extirpate yourself from certain uncomfortablely recognisable traits....

It is indeed a neurosis (we all suffer from!) to split any person "monstrously" into something they are not (a mental construct). Love cures all that.

Leaving all this in the past now, I hope your gorgeous son is enjoying his singular life!