It is always to tricky to break the big news like this. You can say '我怀孕了' -- wǒ huái yùn le

If it is an unwanted pregnancy, you can add '怎么半？' meaning 'what should I do?'. The pronunciation is 'zěn me bàn ?'

If you want to congratulate to a friend who tells you that she is pregnant, you can say 恭喜恭喜 (gōng xǐ gōng xǐ ). It means great, I'm happy for you.