I agree, sooner rather than later. You are already past the first trimester, so you're already over your first personal hurdle. As the previous poster said, your employer should be made aware so that they can plan for the time you will be out and arrange for someone to cover your load. You should also know how long you plan to be out for maternity leave or personal leave. I'm pretty sure that will come up, so it's better to be prepared with an answer.
Here's a bit of mama-wisdome.
As a Mom of 3, I can honestly say second children are tricky. On one hand, you're already a pro with the baby thing, but juggling two kids has to become a more coordinated event. With one child, you can be more flexible, go almost anywhere and take the baby. With 2, you've got 2 nap times, etc. Also when the kids get sick, it's not always at exactly the same time. One child is sick with the other following behind by 2 or 3 days, so everything runs longer. One usually gets the bug worse than the other.
Personality wise, your second child will probably be opposite than your first. Not always, but if you prepare for it now, it won't shock you to your socks when you observe it. If your first was a mellow, easy going baby, your second might well be a screamer, fusser, etc. If you're like my best friend, her first was the fussy high needs baby/child (and still is) and her second one was an easy going, mellow baby.
Sometimes knowing is half the battle. Good luck and congratulations.
K.
Mom to 3 ages 8, 6 & 4