Hey GiGi, I hope you don't mind me jumping in here, this isn't normally I forum I'm on, but saw this and do know a little bit about it.
My understanding is that with twins, amnio is a better choice then cvs. cvs comes with a slightly higher risk (1:300 vs 1:400 with amnio) and my understanding is that with multiples, the results can be less accurate. Amnio when done with twins is really no different then amnio with a singleton except that it's done twice as they draw fluid from each sac individually. The one person I know who did this with twins said it was no harder then when she had had in done with her singleton, just took a little longer. They slowly insert the needle into each sac and will stop what they are doing if baby moves toward it. So it can take longer. Being that the rate of risk with amnio is 1:400, I believe that automatically makes it 2:400 for twins, simply because the procedure is performed twice, but double check with your Dr. on that. If it were me though, I would opt for amnio over cvs in this case.
I think you knew that I had amnio with Ryder? If you want to hear about my experience with the procedure with a singleton, please let me know. Happy to give you the details. And honestly, it wasn't painful at all. Some mild cramping, and then no lifting or anything for at least 24 hours, so make sure you have someone to help you with Marek.
FIrst trimester screen is not innacurate, it's just not diagnostic. All it does is give you the odds. And you are correct, my understanding is that the bloodwork component is not realiable when it is multiples as one can be typical and the other not. When they turn out like mine did (1:13) it can create a lot of unnecessary fear as if your anything like me, you worry, and in my case it was all for naught. Not sure what direction I would take if there is a next time for me, but I can tell you that the amnio really put my mind at ease. I have no regrets.