Tale of a Baby Human

I've never been a math wiz. There are many who can attest to that. But I DO know that 14 weeks does NOT equal four months. But apparently in pregnancy it does. (please reference "What to Expect When You're Expecting".) It's so confusing... first you are automatically 2 weeks pregnant when you first conceive (whaaaa?) and then I pick up another 2 weeks here in the "4th" month? I just don't get it. But whatever, pregnancy mathematics makes it seem like the time is going by faster so it can only be a good thing.