I'm 18 weeks and finally 3 lbs over my starting weight... which is a bit behind, especially since I started out underweight. Not LOTS, but enough that it's tagged as a risk factor for preterm labor on my chart (I was nosy). So I'm stressing the weight thing a bit. My last app I was still 4 lbs under, so I've gone up a bit in the last few weeks and I'm pleased with that, I just want to give these babies the best start I can.
You can do this Elly! Just try to relax, eat as much as you can, and supplement with things like PB and crackers, cheese, and as much protein as you can. I am going to still rave about Boost and what it can do for you, especially as you get bigger and you cannot eat as much food. Extra liquid calories are worth it. Maybe move yourself to 2% milk if you can tolerate it.
Risk for preterm labor is just that and nothing more. Any pregnancy is a gamble when it comes to making it full term. You just never know. I know you know this with your history. Twins aren't really much different other than your risk goes up slightly because your body is stressed more. Just like singletons, you will hear MoMs that made it to 39 weeks with twins and others that delivered at 25 weeks. Who knows why some make it longer than others?
I will say that you are going to need a lot more rest with this pregnancy than your last pregnancies. I would start prepping your kids and DH now for the late second trimester, third tri fatigue. It tends to hit hard as those babies get bigger. I was taking several naps a day at the end. As much as it stinks to feel lazy, I think all the rest I did really helped me manage those final days of the pregnancy.