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To be frank, it's to make sure the baby is somewhat viable. Your chances of a miscarriage are the greatest in the first trimester, so announcing it 'too soon', having something like that happen, and then having to take back the announcement and deal with the grief can be a stressful process. It's ultimately up to you and your partner. The idea of waiting is just 'playing it safe', but it's a pretty big cat to not let out of the bag!
I agree with stefanies. It is also an old wives tale that if you announce the pregnancy to soon that you can lose the baby. But honestly you aren't going to lose the baby unless their are problems. With all 4 of my pregnancies I have told people in the 1st trimester and so far I have 3 healthy boys and I am doing good in my pregnancy now.