What is different between the pre-Mom levels and post-Mom levels?
For one thing, as you have noticed, all monsters will do a full heart of damage, even those that did less in previous dungeons (unless you have the Wafer, which grants damage resistance). This applies to Blood Donation Machines as well.
Some special rooms (the Shop, and the Item Room) aren't present here, but you can still get Arcades and Gauntlet Rooms. The secret room can also contain an arcade machine.
The Womb is also the first area to have bosses from previous dungeons in regular rooms.