What breed? I've had a Down ewe at CS 4.5 lamb twins with no problem at all (and yes, I did try to slim her down!). I've also had Lleyns in perfect condition prolapse to the point where they needed emergency Caesarians. If she's carrying triplets or more she'll mobilise her fat store during the last few weeks of pregnancy, if you don't give her any additional feed (although I'd still recommend a mineral lick to keep up her selenium levels, etc. to ensure a good milk supply). There's still plenty of grass around here but it seems to be about 99% water so of dubious quality!