My ex is drinking during her pregnancy and I want to give her my opinion.
Drinking during pregnancy can be very dangerous. Are you certain that she is drinking alcohol?
I don't know but she was always so irresponsible.
I think before you say anything, you should make certain that she is really drinking, and that she is drinking excessively. A little sip of alcohol won't hurt anything, but if she is drinking regularly, she could really hurt her baby. But, this is a very delicate conversation and should only be had if you are absolutely sure.
So I should just not say anything.
I think you should frame it as you being concerned. You don't want to seem like you're telling her she's a bad person, but that you want what is best for her and her baby. If you accuse her or yell at her, she will be unlikely to respond well.