For two reasons:
1. I am pregnant and almost fainted once a couple of weeks ago (very embaressing!), which I think was due to low blood pressure and therefore wouldn’t have happened if I was more hydrated. It was on a day that I was in the lab alot (can’t drink in there) so it takes more of an effort than just having a big bottle of H20 on your desk.
2. Vitamins haven’t been shown to be beneficial when your pregnant (except for folic acid, which I had). I eat a balanced diet, but want a bit of extra help now that I am in the final three months.