Pulmonary artery or vein problem

Had an ultrasound done at 28 weeks to look at something else and now the dr is saying that the our daughter's pulmonary vein I think is what she said, is too thin and she wants to have an echocardiogram done on her heart to verify if it is or not...... Anyone been told this or gone through this? The dr said she didn't know what it could mean if it is too thin so I have nothing to go off but think the worst.....