Vanishing twin syndrome

At my first ultrasound there were TWO! But one looked empty (the other had a fetal pole and measured right where it should have been).

My doc told me that the second (the one that might just be an empty sac) might develop or more likely, may disappear. Has anyone experienced vanishing twin syndrome? When did it happen for you? Did anything hurt or seem off? My next ultrasound isn’t for another week and would love to hear some experiences if anyone has dealt with this. I’m happy either way - the goal was one healthy baby so if there’s one, fabulous, if there are two, we’ll love them both - just nervous until we get to see an update next week.