How firm should maternity support hose be?

Count Choculitis

I finally broke down and started listening to my mother - who has been nagging me to wear maternity support hose so I don't get (more) varicose veins because of pregnancy.  My legs have felt so heavy and tired, too, that I figure this is worth a shot for reasons other than just cosmetic ones.

I'm about to order a pair online, but I've never ordered pantyhose according to mmHG (milliliters of mercury, I believe).  The different ranges are like 8-10, 10-20, 20-30 and even 30-40. 

 Can anyone recommend which ones to get?  I want to wear these under pants without feeling like my legs are getting strangled by a boa constrictor.  But I want to prevent varicose veins and make my legs feel better.