Why must tell colleagues that you are expecting?

My plan was to let my growing belly self-told the others that I'm pregnant.

But, one day, when I was out with my sister in law (that time I was just a month pregnant), she asked me whether I have told my colleagues about my pregnancy.

And I told her of course not, if I tell, maybe the earliest when I'm at month no. 3. 

Then, she advised me that it is okay not to tell distance relatives or friends first if I don't want to, but to colleagues, it's better if I announce it as soon as possible.

Why?

i. Because if any emergency happened, I would have the privilege to be saved first.

That reminds me, oh yeah, I live in Japan, the very place on earth where all kinds of natural disaster happened (nauzubillah minzalik).
ii. Besides that, pregnancy means my health will be very fragile, I will tend to take emergency leave more often, so by letting them get the head first, will save me and them as well.
iii. Also, telling them sooner, will give them more time to plan for replacement for the time I'm on my maternity leave.

How wise is my SIL. 

So, on the first day back to work, I straight away tell my boss and people around me whom I think would care. Now, my belly is growing like nobody's business while my dream is to have a pregnancy belly like Ivanka Trump. lol xp. I know it won't happen as I am so petite, anything bulging would be easily noticeable.

So, how about you guys? How early did you tell your colleagues that you are expecting?