I was a bit surprised but then delighted when sil announced her third pregnancy aged 46. Not necessarily surprised because of her age, but mostly because she had been taking precautions. She was also surprised, and then delighted.
Different sil had her first and only at 39.
My aunt had her two children aged 42 and 47 after 20 odd years of taking no precautions. What a surprise those two must have been!!
Unprotected sex for 20 years and then suddenly pregnant! Why!?
So in answer to your question, my first thought would be "Yay! Congratulations!"
I'd figure that if you weren't happy about it yourself, you wouldn't announce it to anybody. So I would feel free to enthuse! 50 years ago, women had children in their late forties, and one or two early fifties (that I personally know about) There is nothing new under the sun. 39 is very young.
To analyse it a bit further. I had my last period when I was 59, So I could have become pregnant mid fifties. (Not likely, given the depletion of ovarian eggs and stuff - these are all technical terms) but I COULD have. And I would have been pleased if not delighted.
39 is nowt. Honest. Nobody will turn a hair about somebody being pregnant at 39.
You just have to stop giving a fuck about what other people think.