VOY addresses this in episodes such as "Drone" and "Collective." Long story short, no, the Borg do not reproduce in the conventional sense, expanding their numbers purely through assimilation.

Interesting question. The borg probably grow their population by relying almost exclusively on assimilation. They probably don't reproduce naturally (unless maybe if it's in some kind of extraordinary situation where they need more borg drones and can't assimilate anyone else).

However, whether they ever reproduce naturally, assimilate pregnant women while expecting, or assimilate babies, it seems that babies are just placed into borg maturation chambers until they're fully developed into adult borg drones, so they most likely wouldn't go through a normal gestation or development period.

They expand through assimilation

This is all Voyager era retcons. Originally, the Borg were singular species who presumably reproduced normally, and added cybernetics after birth. This remained the case throughout TNG. While Picard reverts to his previous identity after being disconnected, Hugh and his group remain as they are, suggesting that they've always been Borg. It wasn't until First Contact that the Borg assimilate people en-masse, which leads to Voyager's Borg only being able to reproduce through assimilation. Even then it doesn't make any sense, they have DNA, they should be able to grow new drones! If they must assimilate new species, they should just add the traits to their genetic template and grow drones from that, only altering individuals when they need a lot of bodies that don't have to meet standards.