“How much weight have you gained” - SIL

I have okay relations with my SIL but this just weirdly insensitive. No other text, no other context, just out of the blue asking for my weight.

SIL: “Hi how are you. Baby been active?” Me : “ Yes been kicking. And I’m doing alright” SIL: “How much weight have you gained?”

And she’s not of child bearing age, nor have I seen her recently, so I don’t know how this information is relevant in any way… and there’s no other text after this?

What should I reply? I don’t want to be rude but at the same time I’m quite annoyed.