Hi notdreaming,
I only gave you half the story earlier - I have actually terminated twice for fatal abnormalities (different ones, not related). The chances of what happened to me (ie two apparently random doomed pregnancies) are incredibly low, so I am not telling you this to scare you more. I have since had a healthy child and I already had one before losing the two babies, one at 13 weeks and one as I said at 20 weeks.
In practical terms, a 20 week termination can take a long time - be prepared to wait hours. Mine didn't, and the baby and placenta were delivered all at once, which I think speeded things up. You will be given the choice whether to see/hold your baby or not. We did, because I wanted and needed the connection somehow; others haven't, and I think it's a very personal decision and probably you need to wait and see how you feel. The hospital will take photographs and hand/footprints of the baby too. For a very long time I couldn't look at these, but I am now (nearly three years on) very happy to have them. We also (despite being not religious) had the baby blessed by the hospital chaplain. That felt right to me because he acknowledged her as a person we had lost - as opposed to some sort of medical procedure. The hospital organized the baby's cremation and we attended the service... I can tell you more about all that later though if you want.
For now, you need to get through Monday. I recommend taking magazines, books, distractions, in case it takes ages. I actually found the day less awful than I had feared and less awful than the horrendous scan and all the second opinions, because I did actually get to hold my baby. Afterwards though I was so, so tired and drained; less tearful than just physically and mentally exhausted and shut down somehow. The hospital will allow you to stay as long as you want/need - I wanted to escape, and see DD1 who was at home, yet also felt bad for wanting to leave the baby there; the emotions are very complex and all over the place to be honest and you just sort of get through it. Everyone was very kind, and we were nowhere near labouring women, which does help.
I think I'll stop with the practical details here but do, do pm me if you want to know more. For now, I just want to say that the limbo you are in now is pure hell and I really feel for you. It does get easier, I promise. And you can get pregnant again; the termination itself doesn't affect that at all, but this experience will have altered your perception of pregnancy completely and you will find it much more terrifying. But you will get on with it because you feel the overwhelming urge to be pregnant again. It seems impossible, but it isn't.
PM me for more details, if you like. There is also a support thread here - women who have chosen to terminate - the women who post there all have been in similar positions. I can't link just now but I will later if you can't see it.
Again, be kind to yourself. It isnt your fault, and you couldn't change the prognosis.