This is a fine place for this discussion, although as others mentioned, check with your doctor before going.
It sounds like you will be in some pain wherever you go, so if the doctor says it's still fine to go, being in pain at WDW may not e much worse than being in pain at home.
Personally, I would reschedule to avoid being in pain on my DD's first trip. Since the pain is pregnancy related, you do have an 'end point' for it to plan a different time.
If you decide to go, don't worry about slowing down the other members of your group. If you need to go slower some days, there is really no way around it - as another poster mentioned, not listening to your body could cause problems.
If lying down will help, you can go to First Aid in any park. They have cots in private rooms, cubicles or semi-private rooms, where you would be able to lie down. You do need to check in at First Aid (they need a running count of who is using First Aid and what the reason is for their records).
You probably want to vary what you are doing, so you are not spending long periods doing one thing/in one position. I would suggest alternating short attractions where you have to get in and out with longer ones where you will stay put for a while. You don't know what effect all the getting in, ride for 90 seconds to a few minute, then get out will have on your pain.
The disABILITIES FAQs thread (follow the link in my signature or look near the top of this board) has some posts that will be helpful to you.
Post 23 on page 2 has a list of attractions with warnings for pregnancy - I know you mentioned you are already aware of them, but I have found that people often think some, like Soarin' have warnings when they do not.
And, it's better to know for sure that something has a warning than to think it doesn't.
Posts 18-21 have information about which rides may be difficult to get in and out of. Some of those are gentle rides with no warnings, like Small World, where the ride will not be an issue, but stepping down into the boat (and back out again) may be difficult for you if you are having a painful day.
Post 22 is a list of attractions where guests need to send for a period of time (mostly waiting for the next show) unless they have a mobility device with them.
Post 28 is a list of attractions with moving walkways or stairs, which may be n issue if you are having a bad day.
WDW is a very large place and it's important to realize how much walking is involved per day in a trip to WDW. We just got back and I measured the distance for the first time on this trip - we averaged around 6 miles per day!
If you are not walking that much, you don't know what kind of effect that sort of distance will have on your pain - another reason to take things slow and easy.
Post 2 of the disABILITIES FAQs thread is about renting wheelchairs and
ECVs; you will probably need something during your trip. Post 32 on page 2 is about walkers and some special walkers with old own seats, called rollators.
Each one has their own pluses and minuses. All mobility devices can be parked with he strollers at attractions if you feel like walking. Many people use the mobility device to get from on general area to another, walk around in that area, then use the mobility device to get to the next general area.
So, you don't need to use the mobility device 100% of the time. At theater shows, you can also decide to transfer to a theater seat if you like.
ECVs are motorized mobility devices, so they give more independence and no one would need to push you.
It can be tiring to use one because the driver needs to be watching what is going on at all times - people tend to dart out in front of you and you need to be ready to stop.
There are some attractions that are wheelchair accessible, but not
ECV accessible, so if you are not able to walk in line, you will need to transfer to a wheelchair for those attractions.
If you choose to walk for a while, you will either need to park it or another adult in your group will need to drive it.
If you rent from one of the offsite places, it cn be transported fom your hotel to the parks on most of the WDW transportation. You can also rent a model that can be taken apart for transport in a car trunk.
Wheelchairs that are for rent (off site or from the parks) are generally not set up for the user to propel; they tend to be heavy and the wheels are set farther back than ould be optimal for pushing it yourself.
That means you would be pretty much dependent on someone pushing you to get around. But, you can sit in it and be pushed, walk behind it and use it like a walker for support or park it and walk instead. It sounds like you will probably have enough people in your group to push you.
All attractions are wheelchair accessible to the point of boarding (with the exception of a couple like Swiss Fmily Robinson Treehouse, where guests must be ble to walk).
As with ECVs, wheelchairs cn be transported on WDW transportation. You can also choose to fold it and load it onto the bus like a stroller, folded and held by one member of your group.
Rollators are also able to be rented from offsite places. They can be brought into any lines the same as a wheelchair. With a rollator, you do need to walk, but it does provide support while walking, along with a seat whenever you need it.
May in general is a good time to visit - late enough to miss all the Spring break crowds and early enough to miss Summer crowds. But, as was already mentioned, it can be very hot and dehydration can cause contractions, o you want to watch your water intake.
"Little kid" schedule -up early, play then lunch and an afternoon nap - is also a good scedule for pregnant women. Getting to a park early is also the way to have the shortest waits.