I don't know how much experience the OP has, but IMHO it's probably best for newbies to begin with prepackaged chemicals (powdered or liquid). The reason is that there's plenty to learn with respect to the order in which to use various chemicals, agitation techniques, etc. If you have to learn to mix a developer from scratch, that just adds to the amount that must be learned from the start. Granted, most formulas aren't harder to follow than the average food recipe, but there's still learning involved regarding variant forms of chemicals (anhydrous vs monohydrate, for instance), locating sources of supply, etc. The extras you bemoan in commercial chemicals are there to prevent consistency problems, so between that and the extra complexity in mixing it yourself, I'd expect it'd be harder for a newbie to locate problems with home-mixed chemicals than with commercial chemicals.
Originally Posted by dancqu
This is, however, getting rather far from the OP's stated concerns about chemicals: Safety given her pregnancy. For that, a PC (phenidone/vitamin C) formula has an edge over the more common MQ (metol/hydroquinone) formulas, and not handling powdered chemicals is desirable (although simple PC formulas' dry ingredients are unlikely to be very hazardous). The main safety advantage I can think of to mixing it yourself is that you can compound simpler developers and fixers than are readily available commercially, so there's less chance of having an adverse reaction to something in the chemicals.