Well considering that "it" is used as a dehumanizing transphobic slur when referring to humans. That's why "it" is generally considered offensive terminology when referring to someone's baby when the biological sex of the baby is unknown, born, or unborn. Now before someone gets their knickers in a twist, I have trans friends who have been assaulted by people who attempted to brand, or carve things like "it freak", "it whore", and such in to their very flesh. That level of hateful negativity when a word is used to refer to humans means that as a pronoun for humans, all positive connotation is lost in common parlance.
When it comes to babies whose birth sex I don't know then I'll use "the baby", "[parent's name]'s baby", or "your baby". That avoids any stickiness when it comes to the pronouns of a baby.
With animals? Well "it" is often a passable pronoun, but I find that using "it" for animals can lead to confusion. So I default to "the/your/their [whatever animal it is]", to avoid confusion. Comes from being in environments where there are lots of different animals to refer to, which if you say "it" can lead to confusion about which "it".
