Ketones and nutritional ketosis are totally safe during pregnancy. The only possible unknown is that our bodies don’t normally rapidly increase ketones levels, normally is gradual. Nobody knows if this could effect the fetus (I personally don’t think it is an issue). So it might be best to error in the side of caution. Especially if using Keto OS for general reasons (less cravings, better focus) and not for therapudic reasons (Alzheimer’s, cancer, etc). If using for a therapudic health reason like that, then using it would probably be the better option.