I didn't see the original post, but I will add this:
Even if you DID know your exact delivery date (i.e. it wasn't just estimated), most people are kind of incapacitated for some number of days before and an even larger number of days after...so if you're trying to adjust your due date by a few days so you can attend some event or something along those lines then that's another reason you're dense.
We actually put off TTC by a whole month (when we had already put it off by 6 mo) to be "safe" with regard to my leave policy. I get a good leave if I deliver mid May or later based on how long I've been in my job. We made sure we were TTA until we had a July EDD (and now that and Aug have passed as well but at least we weren't stuck with a situation where we're relying on me delivering on time or late.)