Is it a bad idea to drink green smoothies prior and during the pregnancy? since many people drink it only to lose fat only?

Depends: It all depends on what you put in your PURE green smoothy. Normally it is made up of vegetables ( spinach) and fruits such as banana. If these are the only ingredients thats fine. Avoid parsley. Please do not make this your only nutrition. Just drink it as a supplement to your other meals because, you will neet your fats and proteins etc etc. everything in moderation.