I'm about 16 weeks along with twins and haven't yet told my boss I'm expecting.
The last person she hired into her team (internal transfer from another department) was already expecting when she joined the team. I started my job the first week of April and exactly one week later I took a "work from home day" and had my embryo transfer. So I am almost in the same situation as the last person who joined the group!
My boss and I went on a week-long business trip to London two weeks ago and I didn't want to inform her prior to that trip because I didn't want my health to be a topic of concern. I could imagine her continually asking me if I'm tired or feeling ok, etc.
Complicating matters: I was on vacation this week that just ended and my boss is taking a vacation this coming week. So I am planning to inform her once she's back.
I don't think I'm particularly "late" or "overdue" in telling my employer but my mom, who is a worrywart, can't stop asking me when I'm going to tell. She seems to think that as soon as you pass the 12 week mark you are supposed to tell.