Here’s my opinion (based on personal experience):
First child – tell parents right away. They are sitting on eggshells. See above comments mentioning Rav Chaim Kanievsky’s opinion
Subsequent children – tell parents as soon as the doctor confirms all is well. Saying this unfortunately from experience.
Regarding people other than parents, there is no reason to announce. They’ll figure it out eventually. Your siblings will have heard within 30 seconds of you telling your parents (even if you tell them not to say…)