@anon9000get You have to be calm around your partner. Freak out and worry away from her and come back to her strong and happy. She needs positive and happy people around her.
My water broke at 21 weeks and I held the kid in until 24 weeks. We were in the NICU for 82 days and if you see him now, you'd never know he was a micropreemie.
Those steroids are a miracle and the medicine in NICUs now are so advanced that kiddos are succeeding and thriving more than the research can catch up. Be positive. Preemies are fighters. They have an unknown source of spunk and energy that is amazing and beautiful. My son was able to lift and turn his head enough to pull his breathing tube out at 28 weeks. They routinely find micropreemies moving from one side of the isolette to the other. It is extraordinary what those doctors, nurses, respiratory therapists, and occupational therapists can do. Your baby will be in great hands. And your baby is stronger than you think.
As far as development goes, if there are problems, the resources available now are amazing. You'll have a team of doctors and development specialists there to help. If there are delays, just remember that every child is different, even full term kids develop on a wide spectrum of time. My preemie is 3 now, he had a mild speech delay but is caught up now. No signs of emotional or sensory issues.
Don't think about what could happen, focus on the here and now. You celebrate every successful day your child stays in her belly. Once he or she is here, you celebrate the weight gain and respiratory advancement. It's much easier and better on your anxiety to focus on small success.
You've got this. Your partner's got this. And most of all your son or daughter is going to do it's upmost best to be a Rockstar and kick some ass.