There's still some secrecy -- Enrique captioned the pic, "My Sunshine" ... so, it's unclear if the kid in the pic is Nicholas or Lucy. TMZ broke the story ... Anna and Enrique became first-time parents when she gave birth back in December.

Judging by the baby's clothing ... we're gonna guess this is Nicholas. Hell, we've got a 50-50 shot!

10:55 AM PT -- Ok, now we're more confused. Anna just posted her own shot with one of the twins, and also captioned it, "My Sunshine."

Now, we're guessing Anna's holding the boy, Nicholas -- while Enrique's got the girl, Lucy. We're sure of it! Or, at least 50-50.