Congratulations Messages For Marraige Do You Always Have To Say Congratulations On A Pregnancy?

Do you always have to say congratulations on a pregnancy? - congratulations messages for marraige

I noticed a trend that people always say: "Congratulations" when someone (even a random number Strangers on a message board) when you could be pregnant.

I understand that this is generally the polite thing to say.
However, I have noticed that people always say (or equivalent), if you can not say whther pregnancy is desired or not.

I wonder how it is more appropriate to answer the questions someone when you do not know if the pregnancy is desired, or if you prefer to cancel?
It is a truism necessary if we do not know, do you need?