During my pregnancy, ultrasound found a 3 CM cyst with calcified walls on my spleen. Is this dangerous? How reliable is us in this case?

Not to worry. This is seen occasionally and is likely not a concern during your pregnancy. If you continue to be concerned, you can follow it up after you give birth. Either with another ultrasound or an mri.
I have. Never run into this issue. Are you sure they said the spleen? The cyst isnt unusual but the spleen deal, not sure what to make of that one. I assume u have no medical problems, correct? I'll bet it's an incidental finding of no consequence but dont take my advise; f/u with your doctor. I'm not sure how they saw the spleen on a fetal scan so check to make sure. Good luck.