I think it's obvious in this case. Nelle's baby and the now-deceased baby were definitely not identical. The facial features looked different to me.
And considering that Lucas is a doctor and Brad is in charge of the lab, I foresee Brad having to do a lot of test-fudging if he wants Lucas to continue to believe that's their real baby. Lucas could easily get hold of the baby's original medical records, so something like a different blood type or some condition that's in either Carly's genetic line, the Quartermaine line, or in Nelle's family either showing up or not showing up... well, the whole truth will come out at some point.
I watch on Hulu the following day, so I haven't seen Nelle's baby yet
