From the books, it says it's better to announce after 3-month pregnancy as the fetus is more stable. The chance of miscarriage has dropped tremendously. Therefore, it's time to break out the good news. Of course, the pregnant mother can tell close relatives and friends about her pregnancy during the early stage to share the joy with them.
Keep the good news quiet not soley because of 'small gas' of the baby, but also not to cause great sorrowness and disappointment when miscarriage (touch wood) happens.