I was just looking up the exact same thing. I found an interesting discussion at
midwiferyandmore.com
This was the comment that I found most interesting. I've been taking hydralyte occassionaly to combat some of this summer-heat-pregnancy yukness. I'm taking powder at the moment but the chemist sells it in one litre bottles, as well as icy-poles. Could be an interesting choice!

"I used to work at a birth center here in San Diego.

I prefer to provide an electrolyte replacement drink that is frequently used by marathoners, hikers and laboring women. It's called "Hydralyte" and is available online or at selected sports stores. (I can't post the link but if you google it you will find it--or check for "gookinaid.") The advantage of using Hydralyte is that it's formulated to be extremely similar to the body's biochemistry. You don't, therefore, have to guess at proportions of ingredients or leave things out because they're not available to you (but important nevertheless!). BTW, Gatorade and its knock-offs are not physiologic. (Just witness what happens to marathoners when they attempt to use them during a race!) Just because something says it's an electrolyte replacement drink doesn't mean it qualifies as such under the category of "hard science."

Hydralyte works really well for morning sickness also. Mix exactly per directions to maintain proper concentration and have the woman drink it well-chilled. (Do not add ice or it throws off the formulation.) For pregnant and laboring women, it's best to use the citrus flavors rather than the fruit punch. Hydralyte comes in packets so it can be carried easily in the birth kit. The man who formulated it, Bill Gookin, was a world-class marathoner and has his master's in biochemistry. "Back in the day" when I was also running, and when his drink was in high demand at meets and marathons, everyone called it "Gookinaid" because we couldn't think of anything else to call it!"