How to write a parenthetical plural when the noun pluralizes irregularly?

It's not pretty, but the most common way I've encountered is to list both words, without the parentheses, with either a slash or the word "or" between them:

We were looking at the same baby/babies.

We were looking at the same matrix or matrices.

You can also try "baby (babies)", or reword in some way that avoids the question, or if the context permits, just use the plural.

The simple plural, those formed only by an addition of an "s", is the only case in which the parentheses are used to indicate the indeterminate plurality, as in "poster(s)". In all other cases, the alternatives are both given, separated by an "or", as in "the same baby or babies". (This can also be done for the simple case: "the same poster or posters" is acceptable, and may even be preferred in more formal writing.)