I think it also depends on what you do - if you find yourself being physically active at work then you will notice the difference in what you're capable of in the third trimester. My job is pretty much a desk job, but it is boring me senseless, so I'm going on leave at 36 weeks, theoretically giving me 4 weeks to parrrrrty!
The law here is that you have to tell your boss you intend to take leave at least 10 weeks before you do. By law you only have to give your boss 4 weeks notice of the actual dates you intend to take the leave. It is recommended that you give it in writing and that you also supply a medical certificate (!!) confirming the pregnancy. LOL - I think that's a stupid rule, but there it is.