The one thing women are not told about pregnancy is the complete obliteration of personal boundaries. From the numerous tests at the doctor's office (not counting the many doctors, interns, nurses, etc who will be touching you during labor) to the simple act of walking down the street can make any woman forget that she is more than her burgeoning baby belly.
Perhaps it's that a new person, a new life is inside you that people feel it's their duty to give you advice