Where?
Anywhere you can find room. Preferably the galley or other area where you can prevent the gawkers, but if not possible anywhere the mother is comfortable will do.
The one I dealt with was in Y class with her legs up on the seats, it was "fun" trying to preserve her privacy and fending off the camera phone junkies... ffs...
Many a baby been born in the lavs over the years on flights from the ME... Many unfortunately the result of domestic workers being assaulted by their employers. There's several I recall being found by cleaning crews hidden in the toilets at the end of flights, very sad