I was in a very similar situation. I was given a Dr cert for 100% after 28 weeks (she said that was the normal time to stop working with twins) and that was to be sick leave up until 2 weeks before the birth when maternity leave starts. I had a month off from 16-20 weeks and have been on 50% since 20 weeks due to complications and none of it will affect my maternity leave. (I'm very gratefull for unlimited sick leave here!!!!) I would agree, you don't have to do ANY work.
Good luck, by the way!