Hi A.,
It looks like you've gotten lots of responses, but I wanted to add a midwifery based perspective, which is very different from the medical one. For your review and information-you can make up your own mind.
One thing I studied intensively in my midwifery studies was nutrition, and it's effect on maternal physiology as you progress through the stages of pregnancy. For informational sake, obstetricians very typically do NOT study this topic very extensively, as medical approaches are not oriented towards a "whole body" approach and they have a whole host of medications to use that midwives typically don't. So the focus on nutrition is very extensive with midwives.
First, there is little real evidence to suggest that there is really any such disease as "gestational diabetes". I know that is a very inflammatory thing to say in American maternity society, so I'll back that up...
1) It is normal physiology for a pregnant woman to increase her blood volume by 50% or more during pregnancy. Because of this, and because she is also handling the nutrients for her developing baby, as well as all waste products, etc, the edocrine system goes through enormous changes, especially in the second and third trimesters, including changing pancreatic output of insulin to keep blood sugar levels high longer after eating--this is to give those nutrients and energy to the baby, among other things.
2) The standard for "normal" blood sugars, among other things, was developed in the 50's for male, middle aged patients. NO ADJUSTMENT WAS MADE TO TAKE INTO ACCOUNT THE DIFFERENCES WITH A YOUNGER, PREGNANT, WOMAN! All three of those issues-- younger, pregnant, and female--make a difference. So when you are getting tested for "gestational diabetes", you are being compared to the standards of a physiology that does not match yours at all.
So...let's move on.
It is normal to be thirsty, especially in mid to late pregnancy. Remember that blood volume increase of 50%? Most of that takes place now. You are also needing higher levels of fluids to flush your wastes through your kidneys, as they are processing more because of the baby.
Now, one more thing...and you can do an experiment to prove it if you don't believe me. It may be that you are needing more electrolytes--not just water. That would include salt, citric acid, and potassium. These things, in the correct amounts, are required for your cells and blood vessels to maintain the proper membrane strength so fluids may pass through when they are supposed to and not when they aren't supposed to. Try this. Take a stalk of celery and cut it into three pieces. Get three small bowls or cups and put plain water in one, water with a couple teaspoons of salt dissolved in another, and water with as much salt as you can dissolve in the third. Place the celery stalks with one cut end in the water in each cup and put in your refrigerator overnight. Around lunchtime the next day, take them out and try to bend the celery stalks. You will find the one with plain water (no salt) to be crisp. The one with a small amount of salt will be flexible but not soft. The one with lots of salt should be pretty soft. There is naturally water in the celery, and what you did with the small amount of salt was create a "matching" amount externally to what was internal--and that allows fluids to pass both ways through the outer membrane of the celery. The crisp celery is that way because water can go in, but cannot come out. The mushy celery is that way because water can come out, but cannot go in--the salt is imbalanced either way.
You want your fluids to have some salt, just not too much. The one taste instinct that humans still seem to have is the ability to know by taste how much salt they need--so unless you have really blown your tastebuds by eating super salty foods all the time, feel confident to salt to taste. That way you will get enough electrolytes.
If you have been drinking so much water while you are reducing your salt intake, you are actually diluting your system too much possibly.
An excellent electrolyte drink is Recharge--it's made with natural ingredients so it is guaranteed to be healthy for a pregnant mom.
Finally, remember that this is the time to eat LOTS of proteins to prevent pre-eclampsia. A spoonful of peanut butter between meals, or an extra egg here, cups of yogurt, whatever.
good luck!
Fiora