Swimmers gonna swim. 

Michael Phelps just announced his wife, Nicole, is pregnant with baby #2 -- a little over a year after she gave birth to their first kid.

"Number 2 on the way," the Olympic legend said ... "So excited! Wonder if it's a boy or a girl."

Michael and Nicole got married in a secret ceremony back in June, right after their 1st son Boomer was born. 

Now, they're adding to their brood ... and once again, doing it faster than Ryan Lochte