because statistically, complications are more likely in the 1st timester and if something were to happen, the fewer people we have to tell about it (when they say "oh how are things going with the baby...") the "easier" it will be on us.

i saw it first hand, a girl i knew told everybody in the world she was expecting, she had a miscarriage and for months, people were asking her how things were coming along and it made it all that much more difficult for her.

by only telling our parents/grandparents, it could save us a lot of pain and other people a lot of awkward embarassment if something (God forbid) terrible happens.