Hi Markar,
I'm at 37 weeks now and have kept on my feet and healthy all the way through. It gets exhausting though!
Try reading Elizabeth Noble's Having Twins book. There's lots of information in there about twins and how to keep them in - main points are nutrition, rest and exercise. Nutrition-wise, make sure it's organic and something that will give you energy - whole carbs and quite a lot of protein is good. Once I found out I had twins at 20 weeks I started taking a pregnancy multivit and mineral supplement - the one I used was Viridian's Pregnancy Complex. From what I've read it's worth doing this, just to be sure that you're getting enough. I think attitude is important as well. I've also been going to the osteopath (started off at once a month, then once a fortnight) and the reflexologist.
I was told my iron levels were low at about 30 weeks or so. It was actually 10.7. This is not low but normal for a pregnant woman! The optimal levels are between 9.5 and 10.5 at the end of a pregnancy with a Mean Corpuscular Volume of greater than 84. Your iron levels get low because you produce loads more plasma to carry everything around.
Also try eating 6 meals a day. I found this really hard, but I do try having a snack in between and something protein-ish before bed.
Sorry, this is lots of information, but it's all worked for me to get to where I am comfortably!
Also, remember that the official 37 weeks due date for twins is based on hospital data so doesn't include homebirths and birthing centre information, and it does include all the women who are bullied into inductions at 37 or 38 weeks because the hospital protocol says they should. so personally I think that's a false figure as it doesn't show what a natural gestation period would be. I get a bit feisty about this!
Have a look at the Association of Radical Midwives website as another place to start - I've found many answers on there to questions that I've had. Also worth remembering that however much hospital protocol and advice might scare you now (I found it did), women have been birthing twins ever since ever! And if we didn't birth better than bunnies we wouldn't have taken over the planet quite so successfully!!
I'll stop now... Good luck! Hope this is helpful!
Doris