It can be considered an insult to get the sex of a baby wrong because males are generally considered more valuable than females. It's socially better the refer

It can be considered an insult to get the sex of a baby wrong because males are generally considered more valuable than females. It's socially better the refer to an infant as inanimate than to mistakenly refer to a male child using the female gender. That said, as soon as one knows the correct sex (often indicated by the colour of cloth used in swaddling or clothing, or other markers like design patterns of trucks or ponies, as appropriate) it's important to start using the corresponding grammatical gender otherwise the early indoctrination lacks the enforcement required for a lifetime of identity with arbitrary social constructs like masculinity and femininity. It's not a grammar rule, it's a social rule.