21 weeks- Fluid around baby’s heart?

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So, I had an ultrasound done yesterday at 21.3 weeks. Baby wasn’t too cooperative as she was laying face down and wouldn’t move lol At the end when we saw the Dr to go over everything, we were told there seemed to be a little excess fluid around her heart. The Dr kept using the word “mild” but of course when you hear anything about your baby you become concerned. We are scheduled for an eco next Monday and they also did some blood work on me. Has anyone else had experience or gone through this? Of course I’ve googled everything under the sun but I really don’t know what to make of this, and we are worried!