Probably a fake story, but I almost did that recently.
A former student stopped by with her husband. She had always been skinny and still was but was slouching and had a little pooch. I came very close to congratulating her on being pregnant but thought better of it. In conversation about what she's doing, etc., there was no mention of pregnancy and they are planning on having kids in a few years.
My mother made such comments to my wife early in our marriage (love my mom, but MILs are the worst).
Unless you see a baby actually coming out of the woman, never say anything about pregnancy unless she tells you first.