I wasn't clear from the OP whether you knew your DIL was pregnant. If you did know of the pregnancy, then I cannot see why your DS should not share with you the fact that the scan showed all was as it should be. I can understand your DIL not wanting to share the concern over the low placenta. Her body, her choice. Still, the fact he shared it before he knew her wishes is hardly a hanging offence. If you knew she was pregnant I would express pleasure that all is well, provided your DS tells her he told you all was well. No need to mention the placenta. If you didn't know, then stay silent. She can't hide it forever.
Perhaps her mother had a similar problem in pregnancy and she, the DIL, doesn't want people fussing over her. Relax about it all and enjoy the forthcoming pleasure of a grandchild. Your DS though should explain to her that he wants you to know that all is well.
Be thankful she doesn't come to see you as a low placenta can be a risk for mother and baby. At least you can't be blamed should the worst occur. Sorry to be a bit morbid there but if there is a risk people around her need to know just in case expert help is needed.
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