Hi lulu, I'm expecting twins and I know bed rest is common amongst twin mums because of the greater risk of pre term labour. I haven't been through it myself but I've read a lot about it as I'm a little paranoid about it knowing the risks and having had a scare with lots of contractions at 26 weeks.
Don't know about the FFN test itself and what it looks at or can diagnose, but I do know that if indications of preterm labour are caught early enough then they can keep the babies in for a long time with bed rest and medication. That is what I understand from the books on multiple pregnancies I have, and some of the experiences of twin mums I have spoken with. Don't see why it would be any different for a singleton (but of course I am not a doctor).
It's horrible, and a scary thing to go through. Sorry that you are in this situation.
Other things that might be consoling are that most triplets are born between 29 and 32 weeks apparently, and whilst they spend time in NICU most of them are ok. And when I saw my doctor last week, when I was 30 weeks, he said that if the babies were born from now on they'd probably be ok. Of course it wouldn't be the start I had imagined, and they'd spend a long time in NICU, but at least their long term prospects are broadly ok.