Also, there's a strong social convention that you don't tell people you're having a baby before 12 weeks, because you may well end up actually having a miscarriage or abortion.
This quickly becomes a farce if you stick to it and end up having to be weirdly noncommittal about summer plans.
(I had to tell my dentist "er, the drugs for tooth grinding you're about to prescribe me... I think I'm pregnant..."
"You THINK?"
"Well I was going to buy a test after this appointment...")