Although there's not much wisdom to be had here with regard to what to do with your particular situation, we do have a number of moms here who HAVE gone through POTS pregnancies and childbirths with a pretty wide spectrum of experiences. You may want to start a new post and just put a wide shout-out to all the POTS-moms out there to ask about what their experiences have been as (as I'm sure you already realize) your experience can obviously vary from one pregnancy to another and maybe hearing from other POTS moms can give you an idea of the various ways things may go in more detail. All my experience with kids is from working with them, not having any of my own ... and from BEFORE I came down with dysauto (my hats off to all the moms out there wrangling kids and symptoms at the same time ... oy!!). So while I have info from the "textbook" side of things, I don't even try to profess real "knowledge."
Here's a start at what's been discussed here before:
http://www.medhelp.org/forums/search/266?query=pregnancy
You may be able to directly contact some of the women from those threads if you hover your cursor over their username in blue at the top of a post they made and then click on "send message" in the dropdown menu that pops up. (A tip on that: check the dates of what you're reading first. If it's really old, the person who posted it may or may not still be around our community, so try for more recent posts first. If you're going to reply to something directly in the thread where it is posted, we generally recommended sticking with things no more than 3 months old, 6 months if you want to push it. Past that and things become rather outdated and we consider the thread to be "dead" and still there basically just for reference purposes at this point.)
As for whether you will improve in the second trimester, I hope you do. If hypovolemia (or even just an effective thoracic hypovolemia due to severe pooling of blood in the dependent limbs) is a major component of your POTS—which it sounds to be considering your response to IV fluids—the increase in blood volume normally seen in pregnant women in the second trimester should be beneficial to you. Unfortunately, this isn't one of the texts that I own personally, but from what I can see here it seems to give a pretty comprehensive overview of the cardiovascular changes attendant to the various stages of pregnancy:
http://books.google.com/books?id=UqDEKK4XzT4C&lpg=PA197&ots=PnLeEVwZ_J&dq=blood%20volume%20increase%20second%20trimester&pg=PA192#v=onepage&q=blood%20volume%20increase%20second%20trimester&f=false
(I hope that URL works for you; If you scroll down it should take you to the beginning of the section on the changes in blood volume and then through some other relevant material as well in this preview. One interesting point it mentions is getting relief by lying on the left side in particular; obviously I have no idea whether this might be of help ... I'm guessing you've already tried everything at this point.)
One last thing I thought I would mention is that if your docs haven't tried you with Rx compression hosiery yet, that's something that might be worth a shot. They make ones for pregnant women and it's a fairly common treatment (often used in conjunction with medication or other treatments) for POTS and other forms of orthostatic intolerance in dysautonomia. I wear 40-50 mmHg custom-fitted thigh-high Rx compression hose from Mediven. They are a bugger to get on and off (i.e. it takes TWO people ... me and someone less wimpy), but they do help. If you want more info on those, I can point you in the direction of that.
Take care and keep in touch,
-Heiferly.